Recently the Sloan Digital Sky Survey (SDSS) has completed the largest map of the universe thus far. The map focuses on the positions of quasars. These objects are powered by supermassive black holes in the centers of galaxies, and are so bright they can be seen from the farthest regions of the cosmos. Most quasars are so far away that we have to redefine what “distance” means. In an expanding universe, distance can be defined in a variety of ways. 

For the stars we see in the night sky, their distance is just what you’d expect: the physical distance from the Sun to the star. The bright star Sirius, for example, is 2.6 parsecs away. A parsec is defined by the method used to measure stellar distances, known as parallax. As the Earth orbits the Sun, its view of the stars shifts very slightly. Nearby stars shift more than distant ones, and this is known as a parallax shift. The bigger the parallax, the closer the star. If a star were one parsec away, its parallax would be 1 arcsecond. There are 360 degrees in a circle. If you took a single degree and divided it into 3600 parts, each part would be an arcsecond.

The parallax of nearby stars is small because they are so very far away. While astronomers often use parsecs for distance, a more common measure is the time it takes light from the star to reach us. For Sirius, that is about 8.6 light years, meaning the starlight we observe left Sirius about 8.6 years ago. Of course that distance changes a bit over time. Sirius is moving relative to the Sun. Even if we could travel to Sirius at the speed of light, we would have to make accommodations for its changing location. But this change in distance is small compared to its current distance.

Because stellar parallax is so small, it can only be used for stars out to about 10,000 light years or so. Beyond that the parallax is simply too small to measure. For more distant objects such as galaxies we have to use other methods. One popular method is to use variable stars known as Cepheid variables. Cepheid variables have a particular relation between their overall brightness and how quickly they vary from bright to dim. By watching them vary over time we can calculate their distance. Observations of Cepheid variables in the Andromeda galaxy, for example, shows that it is about 766,000 parsecs away, or 2.5 million light years. Just as with stars, the distance of a galaxy changes over time. Over the course of a 2.5 million year journey to Andromeda, the galaxy would have moved by about 1,500 light years. That’s still a small fraction of its overall distance, but its not insignificant.

With more distant galaxies distance becomes much more complicated. If we measure the motion of various galaxies, usually through the redshift of their light, we find that the more distant the galaxy the greater its redshift. This is due to the overall expansion of the cosmos. Through dark energy, the overall distance between galaxies is increasing, and this cosmic expansion puts a serious wrench in the meaning of cosmic distance.

The most direct quantity we can measure for a distant galaxy is its redshift. Usually this is expressed as z, which is the fractional amount a particular wavelength has changes from its unshifted wavelength. The upper range of z we have observed is about 12, so lets consider a galaxy that has about half that amount, or z = 6. Just how far away is such a galaxy?

Redshift can be caused by two things: the motion of a galaxy through space (often called Doppler redshift) and the expansion of space itself (often called cosmological redshift). We can’t distinguish between them observationally, but we know from various observations that the motion of local galaxies that the Doppler shift tends to be rather small. So its safe to assume that for distant galaxies redshift is almost entirely due to cosmic expansion. To calculate distances, we then have to look at how the universe expands over time, and this relies on which particular cosmological model we use. Typically this is the concordance model, or ΛCDM model, which is your standard dark matter, dark energy dominated universe model. Assuming this model is accurate (and we have lots of reasons to think it is) then we can calculate galactic distances. But we have to be careful about how we define distances.

Suppose we use the parsec definition above. That is, based upon the light we currently see, how far away is a quasar with redshift z = 6? Another way to say this would be “How far away was the quasar when the light left it?” This turns out to be about 1.2 billion parsecs. It’s tempting to convert this to light years, and thus say it was about 3.9 billion light years away, but this is misleading. Because the cosmos was expanding as the light traveled to us, it actually took the light about 12.8 billion years to reach us. So its light travel time distance is actually 12.8 billion light years. This is the most common “distance” used, since it’s easy to compare with the age of the light. When we observe a quasar with a redshift of z = 6, we see the universe as it was 12.8 billion years ago.

Unfortunately this can also make things more confusing. Given that the travel time distance is 12.8 billion years, one might assume that the quasar is about 12.8 billion light years away, rather than 3.9 billion light years when the light left it. But the light we observe was traveling toward us as the universe expanded, while the quasar it left behind moved away from us with the expansion of space. This comoving distance is about 8.4 billion parsecs, which is equivalent to 27 billion light years.

Each of these distances is valid in its own way, even though they are all quite different. That’s why astronomers often stick to redshift.

The post How To Define Distance In An Expanding Universe appeared first on One Universe at a Time.

Astronomers have discovered what seems to be the most distant galaxy yet discovered. The galaxy known as EGS-zs8-1 has a redshift of z = 7.73, trumping the previous record for a galaxy at z = 7.51. Several articles are giving a distance of this galaxy as about 13 billion light years away, but that’s not really an accurate measure, and it’s part of the reason we usually talk about redshift rather than distance.

The redshift of a galaxy is typically given by a number known as z, which is the difference between the observed wavelength of a particular emission line and the standard wavelength as measured here on Earth, divided by the standard wavelength. In this way, an object with no redshift would have a z = 0, and the greater the redshift the greater the z. The nice thing about redshift is that it’s purely an observational result.

From this z number we can infer a distance based upon Hubble’s law, which states that the greater the redshift the more distant the galaxy. But since Hubble’s law also means the universe is expanding, we have to be careful about which distance we mean. Do we want the distance of the galaxy when the light left it? The distance the light traveled to get to us? Or the distance of the galaxy now? These are all different.

With a z of about 7.73, that means the galaxy was about 3.4 billion light years away when the light left it. Because of the expansion of the space between us and the galaxy, it took the light about 13 billion years to reach us. But since then the galaxy has moved away from us at an ever greater rate, so it is now about 29.5 billion light years away from us.

That last distance might seem odd given that the universe is only 13.7 billion years old, but it’s important to keep in mind  that the galaxy hasn’t traveled 26 billion light years in 13 billion years. In fact any actual motion away from us through space would be in addition to the 26 billion light years. That last distance is entirely due to the expansion of space between us. In an expanding universe, distance is always changing.

Which is why astronomers tend to stick with redshift.

Paper: P. A. Oesch et al. A Spectroscopic Redshift Measurement for a Luminous Lyman Break Galaxy at z=7.730 using Keck/MOSFIRE. ApJ 804 L30 (2015)

The post Across the Universe appeared first on One Universe at a Time.

In 1877 Édouard Stephan discovered a cluster of five galaxies in the constellation Pegasus. It came to be known as Stephan’s Quintet. At the time, the quintet was just thought to be a cluster of nebulae. It wasn’t until the early 1900s that we came to understand them as galaxies. We now know that four of these galaxies form a compact group, while the other just happens to be along the same line of sight.

The compact group is one of the most well studied in the universe, and has even been the center of controversies over the nature of galactic redshift. For example, Halton Arp noted that the quintet appeared to be connected through tidal tails implying a gravitational interaction between all five. But the redshift measure of one galaxy is only about 800 km/s, while the other four are around 6,000 km/s. In the standard view, redshift indicates distance, thus the four form a cluster while the other is a foreground galaxy. But Arp thought that redshift was an indication of age, not distance, and he thought the tidal tails of the quintet was strong evidence of his idea.

With more modern observations we can see that there are tidal tails between the four cluster galaxies, but not between the cluster and the foreground galaxy. We also have tons of evidence that redshift is a good measure of distance, and that the universe is really expanding. So the general consensus is that Arp’s ideas are wildly wrong.

Fun fact: the quintet is probably most famous for its use in the opening scene of It’s A Wonderful Life, so it is likely the best known galaxy cluster.

The post Quintet appeared first on One Universe at a Time.

A while back in Nature a paper was published on the most distant confirmed galaxy discovered so far. The paper is behind a paywall, but you can see the arxiv version here. The galaxy, known as z8 GND 5296 has a measured redshift of 7.51. You can see the galaxy in the image above.

So just how far away is this galaxy? It depends on which distance you are talking about.

When determining the distance of far galaxies like this one, astronomers typically give the value purely in terms of its redshift, often known as z. To calculate the z redshift of an object, you look for an emission or absorption line you can identify, such as those of hydrogen. You then compare the observed wavelength of the line from the object with the standard (not redshifted) line. The difference between the observed and standard wavelengths divided by the standard gives you a number known as z.

If there is no redshift, then there is no difference between observed and standard lines, hence the z is zero. Redshift is thus given by a positive number, where the bigger the number the bigger the z. Technically there is no limit to the value z can have, but the highest we have observed is about z = 12. Bigger z also means greater distance. Because of the expansion of the universe, the light of a distant galaxy is redshifted more than the light of a closer galaxy. So the galaxy with the greatest redshift is the most distant.

The reason astronomers usually talk about redshift instead of distance is that the measured z is purely an observational result. Yes, we know that bigger z means greater distance, but the exact distance depends on the model you use for the universe. We know this model is relatively accurate for determining distances, but with z you don’t have to assume any model.

Since this particular galaxy has a z of 7.51, just how far away is it? The first thing you need to do is transform the redshift to the travel time of the light since it left the galaxy. Once the light left the galaxy, cosmic expansion meant the light redshifted while it travelled. Using the standard model of cosmology we can determine the light left z8 GND 5296 about 13.1 billion years ago.

You might think calculating the distance from that age is simple. After all the speed of light is a constant, and if it travelled 13.1 billion years it must be 13.1 billion light years away. But the universe has been expanding throughout its history, so that answer doesn’t work. We can’t even say that the galaxy was 13.1 billion light years away when the light left because the universe expanded while the light travelled. So the galaxy was actually closer than that when the light left.

The distance the galaxy was from us when the light began its journey can be calculated by what is known as the angular diameter distance. For this galaxy that turns out to be about 3.4 billion light years. That means the light from z8 GND 5296 began its journey 3.4 billion light years away, but due to the expansion of the universe took 13.1 billion years to reach us.

To calculate the distance of the galaxy now, you need to start with the fact that it was 3.4 billion light years away from us 13.1 billion years ago and calculate how much the universe has expanded since then. This is known as the comoving distance. This comes out to be about 29.3 billion light years.

So the light from z8 GND 5296 has a redshift of z = 7.51. That means the light left the galaxy 13.1 billion years ago when the galaxy was 3.4 billion light years away. It is now 29.3 billion light years away. That can be a bit hard to wrap your head around.

Which is yet another reason why astronomers focus on z.

The post A Galaxy Far, Far Away appeared first on One Universe at a Time.

Yesterday I talked about apparent sizes, and how Pluto can appear smaller than a distant galaxy, even though the galaxy is much farther away. It turns out, however, that on really cosmic scales apparent size is only part of the story. That’s because the universe is expanding.

In astronomy we generally don’t talk about the distance of far galaxies. Instead we refer to their redshift, often known as z.  The reason for this is that the redshift of a galaxy is pretty unambiguous. You measure the spectrum of a galaxy, then compare a known emission or absorption line with what we observe here on Earth. From the difference between their two wavelengths, we can calculate z.

Calculating angular diameter. Credit: Wikipedia.

Now it is basically true that the bigger the redshift a galaxy has, the greater its distance. But because the universe is expanding, there are different distances that can be defined.  You can, for example, define distance by the time it has taken light to travel from the galaxy to us. We can define distance as how far away the galaxy was when its light began its journey. We can define distance as how far away it is now. So for example, a galaxy with a redshift of z = 3 was about 5.2 billion light years away when the light we observe left the galaxy. The light travelled for about 11.5 billion years, and the galaxy is now about 21 billion light years away. That can be a bit difficult to wrap your head around, which is why we typically just stick with z.

Apparent size vs redshift. Credit: Mark Whittle

Strange as all this is, it has a very real effect on what we observe in the distant universe. When we talk about apparent size, we usually refer to an objects angular diameter. This depends upon its actual diameter and its distance from us.  In a static universe everything would be simple, and the more distant an object is, the smaller its angular diameter would be.

But cosmic expansion changes all that. Since the universe is expanding, distant objects will appear to increase in size. The object isn’t getting larger, but as the universe expands the light traveling from a distant galaxy appears to spread out a bit. For closer galaxies this isn’t significant, but for distant galaxies it is.

For close galaxies, the greater the distance, the smaller their apparent diameter, but around z = 1.5 cosmic expansion becomes a bigger factor than the galaxy’s distance. As a result, galaxies with higher redshifts actually start appearing larger.  The most distant galaxies can appear significantly larger than closer galaxies. This doesn’t mean that distant galaxies are actually larger, simply that they appear larger due to cosmic expansion.

What’s particularly interesting about all this is that we can use this effect to determine the way in which the universe is expanding. This is part of the reason we know the universe contains dark matter and dark energy.  But that’s a story for another day.

The post Closer Than They Appear appeared first on One Universe at a Time.

Ever since Henrietta Leavitt discovered the period-luminosity relation for Cepheid variable stars, and Edwin Hubble used her work to demonstrate the relation between the redshifts of galaxies and their distances, we’ve had a pretty good idea that the universe was expanding.  Since then we’ve gathered much more evidence on the connection between redshift and cosmic expansion, including redshift observations of distant supernovae that show the universe is undergoing cosmic inflation due to dark energy.  While cosmic expansion is now well established, there have been some interesting mysteries along the way. One of these involves some seemingly strange behavior of quasars.

Quasars were originally discovered as bright radio sources that appeared almost starlike. In fact the term quasar is derived from quasi-stellar radio source. We now know that quasars are powered by supermassive black holes, and are known as active galactic nuclei (AGNs).  Because of their small apparent size, their distances can only be determined by their redshifts.  This can be difficult to do, because the light they generate is produced near the black hole where things like gravitational lensing and gravitational redshift can occur. The spectral lines used to measure redshift can be swamped by the overall intensity of the quasar, making them difficult to analyze.

Spectra from different quasars compared. The spectra redshifts, but the pattern remains the same. Credit: NOAO

Then there are strange cases like AO 0235+164, which is a quasar with a faint emission line with redshift z = 0.94, but an absorption redshift of z=0.524. It was results such as these that led some to wonder if perhaps the whole cosmic expansion idea might be wrong, or at least not quite as clear as we might think.  In the late 1960s, Halton Arp made a survey of irregular galaxies and noticed that for about a hundred of them there appeared to be an associated quasar.  In each case the quasar had a redshift that was much larger than the associated galaxy.  This led Arp to propose that perhaps quasars are objects ejected from galaxies at very high speeds. Then in the late 1970s William G. Tifft observed that redshifts of galaxies appeared to have a periodicity to them, as if redshifts were somehow quantized.  Based upon these results, Arp and others argued that redshift cannot be due to cosmic inflation, but must be due to some intrinsic process.

If redshifts were quantized, we’d see concentric circles here.
Credit: Sloan Digital Sky Survey.

At the time when these ideas were proposed they were worth considering. The evidence for ejected quasars was plausible, and even quantized redshift (though controversial even then) appeared to have some evidence to support it.  But over the next 40 years we gathered more observational evidence.  A lot more. Arp’s original quasar-galaxy connection was based upon about 100 galaxies.  We now have large all sky surveys where we’ve measured the positions and spectra of a lot of galaxies and quasars.  The Sloan Digital Sky Survey, for example, now has a database of more than 930,000 galaxies, and 120,000 quasars. We’ve also greatly increased our ability to analyze correlations and periodicities.  What used to take months to analyze can now be done in an hour on a laptop computer.

Distribution of galaxies. The solid line of random distribution matches observation. The dotted lines of ejected quasars does not. Credit: Tang and Zhang

What we found was that the ideas of Arp and Tifft don’t agree with observation. What once hinted at redshift quantization is now seen to be due to a clustering of galaxies.  When large number of galaxies or quasars are analyzed, the quantization pattern fades. We’ve also found the scale at which galaxies cluster matches the clustering prediction of cold dark matter. The idea that quasars are  ejected from galaxies also doesn’t match the distribution of quasar redshifts. A recent study published in the Astrophysical Journal looked at both quasar periodicity (redshift quantization) and ejected quasar using the SDSS database, and found that neither matched observation.

And AO 0235+164? Turns out that it’s a quasar behind a closer galaxy.  The emission line with a higher redshift is from the quasar, and the absorption line with a lower redshift is from the closer (foreground) galaxy.  There are several similar examples, and many of them also show gravitational lensing of the quasar’s light around the galaxy.

So both of these models have been largely rejected.  With nearly a million measured galaxies and quasars, it has become clear that the one model that best matches the data is an inflating universe with dark energy and cold dark matter.  There are still a few researchers who strongly disagree. Their work sometimes get published in a peer reviewed journal, and that’s fine.  It’s always good to have a few dissenters pushing to keep the rest of us honest.

Of course there is another pattern that has arisen, and that is the one where every time somebody writes about how quantized redshift and Arp’s non-inflationary universe model doesn’t match the data, a flood of amateur commenters hit your page to declare how wrong you are. They troll your comments and send you angry personal messages. They’ll post link after link to other papers, and demand you go through each one in detail.  When you don’t accept their view they accuse you of bias and closed mindedness.

Which is why every time the topic comes up, it has astrophysicists seeing red.

Paper: Su Min Tang and Shuang Nan Zhang. Critical Examinations of QSO Redshift Periodicities and Associations with Galaxies in Sloan Digital Sky Survey Data. ApJ 633 41 (2005) doi:10.1086/432754

The post Seeing Red appeared first on One Universe at a Time.

There’s a new paper in the International Journal of Modern Physics which presents evidence that the universe is not expanding. You heard that right. If true it would overturn decades of cosmological theory. It’s the kind of revolutionary find that wins Nobel prizes. It’s gotten a bit of attention in the popular press, but don’t throw your old astronomy books out just yet.

The paper focuses on a cosmological test known as the Tolman test.  It was first proposed in the 1930s by Richard C. Tolman, and it is done by comparing the surface brightness (brightness per area) galaxies to their redshifts. If the universe were perfectly static, then the surface brightness of a distant galaxy should be the same as that of a close galaxy.  This is because while distant galaxies appear dimmer, they would also appear smaller by the same amount, since apparent brightness and apparent area both follow an inverse square relation. Of course, we know that most galaxies have redshifts, and this would serve to dim a galaxy even further. But it turns out that the amount of extra dimming is proportional to the redshift. So for a static universe with redshift, the ratio of surface brightness to redshift should be constant.

The Tolman test has been done several times before, with thousands of galaxies, and the results have agreed with the expanding universe models. So what’s different about this paper? It doesn’t look at a traditional static universe where redshift is due to the motion of galaxies relative to us, but rather an alternative static universe where light “ages” by some unknown mechanism so that it redshifts over time.  In this way, distant galaxies have a redshift due to their “tired light” rather than relative motion or cosmic expansion. For such a model, the surface brightness should dim proportional to redshift.

So the authors compare the surface brightness and redshifts of both low redshift and high redshift spiral galaxies, and find that the results agree with their static tired light model (what they call the SEU model).  They then conclude that while the result is not in itself sufficient to overturn the standard expanding universe model, it shows that more research should be done on the SEU model.  Hence the headlines “Universe is Not Expanding After All, Scientists Say”.

But not so fast.

If you’re going to test an alternative model that requires the introduction of some unknown mechanism for redshift, you should probably compare your results to an expanding universe model to see if yours works better. In particular, you should probably compare your data to the ΛCDM model (the standard dark energy/dark matter/expanding universe).  Do they do this? No.  In their own words, “In this paper, we do not compare data to the ΛCDM model. We only remark that any effort to fit such data to ΛCDM requires hypothesizing a size evolution of galaxies with z.”  Apparently hypothesizing a size evolution for galaxies is bad, but introducing an unknown tired light mechanism to preserve a static universe is okay.

There’s another reason they didn’t compare their results to ΛCDM, and that’s because it would match the data equally well.  Such comparisons have been done before, and they support ΛCDM.  In fact, a paper earlier this year used a similar cosmological test known as the Alcock-Paczynski test, and compared the results to a range of models. The test eliminated all but two models, ΛCDM and static tired light.

Of course tired light is excluded by other observational evidence. If we lived in a static tired-light universe, distant galaxies should appear blurred (they don’t), distant supernovae shouldn’t be time-dilated by cosmic expansion (they are), and we shouldn’t see a cosmic microwave background (we do).

So does this paper show that the universe might be static? No. They didn’t compare the results to a range of static and expanding universe models to show the static model works better. They assumed a fringe static model specifically designed to match the standard expanding model.  They even state this in their paper: “In this paper we reconsider this subject by adopting a static Euclidean Universe with a linear Hubble relation … resulting in a relation between flux and luminosity that is virtually indistinguishable from the one used for ΛCDM models.” In other words, they chose the tired light static universe model because it matches the standard expanding universe model for the Tolman test.

This is not how science is done.

Paper: Eric J. Lerner et al. UV surface brightness of galaxies from the local Universe to z ~ 5. Int. J. Mod. Phys. D, (2014). doi: 10.1142/S0218271814500588

Paper: López-Corredoira M. ALCOCK-PACZYŃSKI COSMOLOGICAL TEST. ApJ. 2014;781(2):96-.

The post Selection Bias appeared first on One Universe at a Time.

The origin of the universe is often portrayed in popular science as a vast sea of darkness.  Centered in this darkness is a bright point of light, which suddenly expands, filling your view with light, fading into a dance of galaxies.  Of course this raises all sorts of questions:  What did the universe expand into? What triggered the initial explosion?  Where did all that matter and energy come from? The problem is, this isn’t how cosmologists see the big bang at all.

Popular science loves to portray cosmology starting with the big bang and ending with our modern universe, but in astrophysics we work the other way around.  We start with what we observe in the universe today, and work back as far as we can to the early moments of the universe.  This is an important distinction, because it means we don’t have to know every detail of the origin of the universe to know quite a bit about its history and early period.  In the same way, biologists don’t have to know exactly how early life appeared on Earth (abiogenesis) to know that the variety of life we see today evolved through natural selection from that early common ancestor.  Understanding that earliest moment is our destination, not our starting hypothesis.

So let’s walk through the process of how what we observe today leads us to conclude that the universe started with a big bang 13.8 billion years ago.  It’s a rather detailed process, but I’ve written about many of the underlying topics before.  Instead of restating all of them here, I’ve just summarized them and added links to earlier posts).  How far you want to delve into the details is up to you, but I think it’s useful to explain just how well we understand the origin and history of the universe.

The first clue was found by observing the relation between a galaxy’s distance and speed. There are various ways we can determine the distance of a particular galaxy.  For example, there are certain variable stars known as Cepheid variables that brighten and darken at a rate correlating to their overall brightness.  There is a type of supernova called type Ia that has a pretty standard brightness.  By observing the apparent brightness of these things in a particular galaxy we can get an accurate measurement of just how far away they actually are.

We can measure the speed of a galaxy by observing the Doppler shift of light coming from the galaxy.  Atoms and molecules emit and absorb light at specific wavelengths.  By observing this spectral pattern we can determine what type of atoms and molecules exist in a particular galaxy.  But if a galaxy is moving away from us, that pattern is shifted a bit toward the red end of the spectrum.  The light waves emitted from the galaxy are stretched a bit due to the galaxy’s motion away from us.  Similarly, if a galaxy is moving toward us, the pattern is shifted a bit toward the blue end of the spectrum, as the light waves bunch up a bit.  You’ve probably experienced this effect with sound, where the sound of a passing car or train sounds higher as it approaches you and lower as it passes you by.

When we observe different galaxies, we find that the light of most galaxies are red shifted.  Not only that, the more distant a galaxy is, the more its light tends to be redshifted.  This was first observed in detail by Edwin Hubble in 1927.  Hubble demonstrated that there was a linear relationship between a galaxies distance and its redshift.  When this was first observed, it was generally thought that the universe was pretty static. If that were the case, then one would expect galactic speeds to be random, with some moving toward us, and some moving away from us.  Since galaxies appear to be receding from us at a rate proportional to their distance, a better model is that of an expanding universe.  Not just fixed region of space where galaxies are flying away from us, because that wouldn’t account for more distant galaxies having greater speeds.  Instead it must be that the universe itself must be expanding, kind of like bread dough rising.

That seems like a rather radical idea (which it was), but it is the model that best fits the data.  It also agrees with Einstein’s theory of relativity, which has been verified extensively.  Einstein had actually had a chance to predict the expansion of the universe, since it’s a consequence of the theory of relativity.  But Einstein assumed the universe must be static, so he introduced a cosmological constant to allow for stationary universe.  More modern observations show that not only is the universe expanding, it is expanding at an ever increasing rate, and one way to account for this is through a cosmological constant.

So if the universe is currently expanding, then in the past the universe must have been smaller.  Extrapolating really, really far back, the universe must have been really, really small.  So it must have had a beginning as a small initial “seed”.  This idea was first proposed by Georges Lemaître, who referred to that initial seed as the primordial atom.  From Hubble’s original data you could get an age for this primordial atom of 10 – 20 billion years, which would be the age of the universe.

Now this is a huge leap.  After all, no one looks at a loaf of rising bread and presumes a week ago it must have begun as ultra dense “primordial dough”.  Many astronomers thought extrapolating cosmic expansion back to a primordial atom was pseudoscientific nonsense.  Among them was astronomer Fred Hoyle, who actually coined the term “big bang”.  (It’s rumored that Hoyle meant the term to mock the idea, but Hoyle denied it.)  Hoyle proposed an alternative interpretation, known as the steady-state model.  In Hoyle’s model, the universe has a process of slow continuous creation of matter, which creates the positive pressure necessary to cause cosmic expansion.  Thus the universe is ever expanding, but is ageless.

Of course both of these models have very clear predictions.  In particular, the big bang model predicts a very specific signature.  If the universe began as a dense primordial fireball, then a remnant of that intensely hot period must still exist.  As the universe expanded its temperature would cool, but it wouldn’t be zero.  So either there is a background temperature to the universe, or the big bang model is wrong.  Given Hubble’s observations of cosmic expansion, that temperature should be a few Kelvin today.  In 1965 just such a background temperature was observed by Penzias and Wilson.  This cosmic microwave background as it now known matched the temperature of a thermal blackbody exactly, with a temperature of 2.7 K.

The cosmic microwave background (CMB) and the evidence of cosmic expansion demonstrated pretty clearly that billions of years ago the universe was a primordial fireball.  But we have to be a bit careful here.  The simple existence of the CMB does not tell us the universe began as a primordial atom.  The CMB is not light from the big bang itself, but light from when the universe had a temperature of about 4000 K.  At higher temperatures hydrogen ionizes into a plasma of electrons and protons.  Light is heavily scattered in a plasma, so it isn’t possible for us to see anything further back than then.

Light travels at a finite speed (about 300,000 km/s), and that means the more distant an object is the longer it takes for the light to reach us.  That means when we view distant objects such as galaxies, we are seeing them as they were in the past.  It also means when we observe something that happened in the past, we observe how it happened light years away from us.  Our best measurement of the age of the CMB is 13.798 billion years ago.  That means the CMB we observe is from a region of space that was 13.798 billion light years from our current position at that time.  Due to the expansion of the universe, that region of space is about 47 billion light years from us today.  In other words, by the time the universe had cooled enough to condense into neutral gas (the time of the CMB), that gas covered a region at least  28 billion light years across, because that was the size of the observable universe at the time.

So now that we know that 13.8 billion years ago the observable universe was a primordial fireball 28 billion light years wide, what’s to say we can extrapolate further back than that?  What if the universe simply began as a fiery expanse of gas?  To go beyond the time of the CMB we need high energy physics.

We know high energy physics pretty well.  We’ve been doing high-energy experiments since the mid 20th century, so we have a good understanding of how matter behaves at high energies.  If we use this knowledge to extrapolate before the CMB, we reach a point where the temperature would be about a billion Kelvin, too hot for atomic nuclei to form.  If the universe began at least that hot, then as it cooled the protons would collide with such energy that a fourth of them would fuse into helium nuclei, a process known as nucleosynthesis.  That means the matter of the post CMB universe would have to consist of about 75% hydrogen and 25% helium by mass (with small traces of elements such as lithium).  This ratio agrees perfectly with the distribution of elements we see today.  We have recently observed the spectra of distant quasars and observed early gas clouds that contain no higher elements (carbon, nitrogen, etc), exactly as we would expect from nucleosynthesis.

Once we’ve reached back to nucleosynthesis, we’ve covered the history of the universe back 13.8 billion years to the time where the initial elements of the universe formed.  At this point the universe is at most about 10 seconds old.  We can extrapolate further back using particle physics, to a time when quarks and gluons form into protons and neutrons, or earlier, where the weak nuclear force and electromagnetism unite into the electroweak force. At this point the universe is no more than a trillionth of a second old, and the observable universe is about the size of a grapefruit.

This is what particle physics, astronomy and astrophysics tells us.  This is what we can demonstrate scientifically.  The early observable universe was once small enough to fit in the palm of your hand.

Of course this is only the observable universe.  Remember that our view is limited by the finite age of the universe.  If that early universe were truly only the size of a grapefruit, then its mass and energy would have curved space over time, and we would see the effects of that curvature on the expansion of the universe.  But to the limits of our measurements the universe has no overall curvature.  That means the universe must be much larger than the region we can observe.  As best we can tell, the universe is infinite in size. So our best understanding of the universe is that it’s infinite in space, finite in time, made of matter, dark matter and dark energy.

And we are a part of it all.

The post Primeval Atom appeared first on One Universe at a Time.

A starburst galaxy is a galaxy that’s producing stars at a very high rate.  The rate of star production is so high that it would use up the available gas and dust in a time period much shorter than the typical age of a galaxy.  While the age of a typical galaxy is on the order of billions of years, a starburst galaxy would use up all the available dust in about 10 million years.  This means a starburst galaxy must be in a galaxy in an active phase of star production, rather than being a separate type of galaxy.  We often see starburst galaxies in the midst of a merger or close encounter with other galaxies, which would indicate that starburst periods can be triggered by galactic collisions.

There is observational evidence that starburst galaxies were much more common in the past, which implies that young galaxies often enter a starburst period in their youth before settling down to a more steady rate of stellar production.  But how do you verify such an aspect of galactic evolution when when it occurs on a scale of billions of years?

The trick comes from the fact that the universe is expanding, and that we can observe galaxies very, very far away.  Because of the expansion of the universe, more distant galaxies appear to be receding from us faster than closer galaxies.  The more quickly a galaxy recedes, the more its light is redshifted due to the Doppler effect of light.  This means we can gauge the distance of galaxies by their redshift.  This redshift factor is typically given in a quantity known as z.  This z is zero if there is no redshift, and gets larger as the redshift increases (and therefore is at a greater distance).  By measuring galaxies at greater redshifts we therefore observe galaxies at greater distances.  Since it takes light longer to reach us from those galaxies, we are effectively looking at galaxies from further in the past.

As recently published in Nature a team used this very method to look at the evolution of galaxies.  Of course to look at starburst galaxies, the team needed a method of distinguishing starburst galaxies from regular ones.  To do this they used an effect known as gravitational lensing to identify so-called dusty starburst galaxies.  Gravitational lensing occurs when light from a more distant source is deflected by the mass of a closer galaxy.  Basically the light is bent by the mass of the galaxy, which distorts the background light.  In the figure below you can see an image of several galaxies in gray with the gravitational lensing effects shown in red.

Gravitational lensing by starburst galaxies. Credit: J. D. Vieira, et al.

Dusty starburst galaxies are ones that are actively producing stars but still have a great deal of dust and gas in them.  As a result they are relatively bright for their size, but also are massive enough that they tend to gravitational lens background light in a measurable way.  It is unusual to find a dusty starburst galaxy that does not also have a gravitational lens effect.  So the team looked for relatively distant galaxies of a certain brightness that demonstrated gravitational lensing, which they could identify as starburst galaxies.  They then measured the redshifts of these galaxies, determining their distances.  By looking at the distribution of starburst galaxies as a function of redshift they could determine how the number of starburst galaxies changed over time.

What the team found was that starburst galaxies were much more common in the past.  About a thousand times more common in the early universe than today.  It would seem then that early galaxies often entered a starburst period of very active star formation.

The post Dusty Starburst appeared first on One Universe at a Time.

While they were once a mystery, we now know quasars are driven by black holes in the center of galaxies, and are part of a larger class of objects known as active galactic nuclei, or AGNs.  What makes quasars distinctive is that they are very bright, and their light is highly redshifted.  This latter property means that they are also very far away.  Because of their great distance, the light from quasars began their journey billions of years ago.  This means the study of quasars allow us to understand the earliest examples of active black holes.  The great distance of quasars also poses a challenge, because it is difficult to measure the properties of an object billions of light years away.  But sometimes a bit of luck will allow astronomers to make some good observations.  Such is the case of a recent paper in Nature which measures the rotation of a quasar’s black hole.

The lensed x-ray light from the quasar. Circles indicate regions used in the analysis. Credit: R. C. Reis, et al.

The particular quasar in question has a redshift of z = 0.658, which means its light left the quasar about six billion years ago.  It also happens to be behind a much closer galaxy from our viewpoint.  You might think that might make observing the quasar worse, but in away it is actually a good thing. The mass of the close galaxy acts as a gravitational lens, bending the light of the quasar a bit, and focusing it in our direction.  This means we actually see more light from the quasar than we would if the closer galaxy wasn’t there.  Of course the galaxy also distorts the light coming from the quasars, so the team had to reconstruct the image of the quasar using the gravitationally lensed light.

Doing that, they then had a strong enough x-ray source from the quasar to analyze its rotation.  This is done by looking at light reflected off its accretion disk.  The region around the supermassive black hole of the quasar generates intense x-rays, and some of it reflects off material in the accretion disk.  The motion of the accretion disk around the black hole means that some of the reflected light is redshifted more than the average for the quasar, and some less.  By measuring this difference it is possible to measure the rotation of the accretion disk, which in turn allows us to determine the rotation of the black hole.

The rotation of a black hole is often given as a parameter known as “a”.  This parameter can have a value between 0 (no rotation) and 1 (maximum possible rotation).  Sometimes in the popular press it is stated that the maximum rotation rate is the speed of light, but that isn’t quite how things work.  So we’ll stick with “a” as a measure of its rotation.  What the authors found was that this particular quasar had a rotation of at least a = 0.66, and very possibly as high as a = 0.87.  This is a very high rotation rate, and it likely means this particular black hole formed as a merger between two smaller black holes.

What makes this interesting is that it hints at early supermassive black holes forming by mergers.  By the time the universe was 7 billion years old, two supermassive black holes had formed, then merged to produce this fast-rotating supermassive black hole.  Of course this is only one example, so we don’t know if such fast-rotating quasars are common or exceptional.

What we do know is that even early black holes can rotate very fast indeed.

Paper:  R. C. Reis, M. T. Reynolds, J. M. Miller & D. J. Walton. Reflection from the strong gravity regime in a lensed quasar at redshift z = 0.658. Nature. doi:10.1038/nature13031 (2014)

The post Turn, Turn, Turn appeared first on One Universe at a Time.

There’s a cosmological model that has gained popularity on the internet known as the Electric Universe.  The basic claim of the Electric Universe model is that much of the astronomical phenomena observed in the universe is driven by electrical interactions rather than gravitational ones.  Proponents of the model claim that the Electric Universe is a much simpler solution that solves many of the cosmic mysteries mainstream astro-scientists are unable to solve.  The model is so simple that it doesn’t require any of that mathematical obfuscation found in the standard model.  But astro-scientists are too set in their ways to look at the model with an open mind.  We certainly can’t ignore such a revolutionary idea, so let’s put it to the test.

There are actually many variations to the Electric Universe model, but the most popular version seems to focus around the book by Thornhill and Talbot listed below.  It is this basic model I’ll discuss here, using the references listed at the bottom of the post.  If you want to get an overview of the model, Findlay’s ebook (available for free) is as good a reference as any.  The basic idea of this particular model is that cosmic magnetic fields interact with interstellar plasma to drive astrophysical processes.  Gravitational interactions play a negligible role in the universe.  From this idea several claims and predictions are made.  In particular:

Neither dark matter nor dark energy exist.  Black holes don’t exist. The big bang didn’t happen.

Galaxies are formed by kinks in cosmic magnetic fields.  They begin as electric quasars which then expand into modern galaxies.

Stars are electrically charged masses formed within galactic plasmas.  They are not heated by nuclear fusion within their core, but rather by a flow of plasma, similar to a florescent light.

Stars “give birth” electrically to companion stars and gas giant planets.

Redshift is not a measure of galactic distance.  It is instead a measure of galactic age.

Special Relativity is wrong.  General Relativity is wrong.

A neutrino image of the Sun. EU predicts this doesn’t exist.
Credit: R. Svoboda and K. Gordan – LSU

So, where to begin?  Let’s start with the Sun.  In the standard model, the Sun is powered by nuclear fusion in its core.  There the fusion of hydrogen into helium produces not only light and heat, but neutrinos.  In the electric universe model, the Sun is lit by electrically excited plasma.  This gives us two very clear predictions.  The first is regarding neutrinos.  The standard model predicts that the Sun will produce copious amounts of neutrinos due to nuclear interactions in its core.  The EU model predicts the Sun should produce no neutrinos.  The EU model clearly fails this test, because neutrinos are produced by the Sun.  We have not only observed solar neutrinos, we have imaged the Sun by its neutrinos.

The second prediction regarding the Sun can be seen in its spectrum.  In the standard model, the nuclear reactions in the Sun’s core produce light and heat that cause the star to shine.  If this is the case, then Sun should emit thermal radiation.  In other words, the spectrum of colors its gives off should be an almost continuous, with dark lines where cooler gasses in its upper atmosphere absorb some of the light.  If instead the Sun were lit by electrically excited plasma, as the EU model predicts, the spectrum should be a discontinuous spectrum of bright lines.  Plasma discharges do not emit a continuous spectrum of light.  Of course, what we see is a continuous spectrum as the standard model predicts.  Once again, the EU model fails.

Top: The nearly continuous spectrum of the Sun. Bottom: The bright line spectrum of a compact florescent light. Credit: John P. Beale

Unlike the neutrino observations, the solar spectrum has been well observed since the 1800s.  Long before the EU model was ever proposed.  It is a test you can do at home with a diffraction grating.  Beyond any shadow of a doubt, the Sun gives off a thermal spectrum, not a plasma one.

But lest we be accused of not giving the Electric Universe model a fair shake, let’s look at the other claims.  Are special and general relativity wrong?  Nope.  They’ve been confirmed in the lab.  In fact whenever you use your mobile phone’s GPS to find a local coffee shop, you’re communicating with satellites that correct for the effects general and special relativity.  Relativity is not merely abstract theory, it is now applied technology.

How about the idea that stars “give birth” to other stars and planets?  If that were the case, we should see stars form as isolated objects in stellar nurseries, then later form planetary systems.  Instead, what we see is protostars form with protoplanetary disks of gas and dust out of which planets form.  We’ve observed these at various stages of development around different stars, and even have dozens of examples in the Orion nebula, which is a nearby stellar nursery.

Protoplanetary disks seen in the Orion Nebula. Credit: NASA/ESA and L. Ricci (ESO)

It doesn’t look good for the Electric Universe model.  But let’s give it one last chance.  In the standard model galaxies form gravitationally, and are well developed relatively early in the universe.  Quasars are powered by black holes in the center of galaxies, and are one example of what we call active galactic nuclei.  In the EU model, quasars are formed by pinches in cosmic magnetic fields, and from them galaxies form.  Rather than being an indication of distance, redshift is a result of the age of a galaxy or quasar.  So as galaxy matures, its redshift decreases.  If the EU model is right, then we should only see quasars with high redshifts (therefore large inferred distances).  Also, the more distant (redshifted) a galaxy, the less developed it should appear.

So here’s a collection of barred spirals at different distances (or redshifts).  Notice how the most distant ones are the least developed?  No?  Actually they all look pretty similar, which is exactly what the standard model predicts, and what the EU model says absolutely shouldn’t happen.  By the way, the nearest quasar observed (3C 273) is only about 2.4 billion light years away, which means it has a smaller observed redshift than three of these fully developed galaxies.  Again in complete contradiction to the EU model.

So never let it be said that an astro-scientist has never considered the electric universe model with an open mind.  The Electric Universe model is wrong.  Provably, clearly and ridiculously wrong.

We’ve put the Electric Universe to the test.  Final Grade:  F-

Reference: The Electric Universe by Wallace Thornhill and David Talbot

Reference:  The Electric Sky by Donald E. Scott

Reference:  A Beginner’s View of Our Electric Universe by Tom Findlay (PDF)

The post Testing the Electric Universe appeared first on One Universe at a Time.

When measuring the motion of distant galaxies, we use the Doppler effect to measure their speed relative to us.  Basically, as a galaxy moves away from us, the light from the galaxy appears more red than it actually is.  This is similar to the way the sound of a train can sound lower as it moves away from you.  Of course things aren’t quite that simple because the Universe is also expanding.  This means that the redshift of a galaxy is partly due to its motion relative to us, and partly due to cosmic expansion.  This cosmic expansion is known as the Hubble flow.

Galaxies of the Virgo cluster have motions relative to the Hubble flow. Credit: Brews ohare

The Hubble flow is what makes more distant galaxies appear to be moving away from us more quickly than closer galaxies.  For distant galaxies the Hubble flow becomes the main part of its redshift.  But the gravitational interactions of galaxies can give them motions that add or subtract to the Hubble redshift.  One example of this can be seen in Hubble’s original data, where the redshifts of galaxies in the Virgo cluster clearly differ from the overall Hubble relation.

The cosmic microwave background shows our relative motion. Credit: WMAP.

We can also measure the Hubble flow in other ways.  Observations of the cosmic microwave background and gravitational lensing allow us to determine the rate of cosmic expansion as well.  This means we can use the redshift to get an idea of how galaxies move relative to each other.  We can also use the cosmic background to determine that the Milky Way is moving about 370 km/s relative to the Hubble flow.  So when we measure the redshift of other galaxies we need to take that into account.

When studying cosmology, sometimes it helps to go with the flow.

The post Go With the Flow appeared first on One Universe at a Time.