We’ve known for nearly a century that the Universe is expanding. The fact that galaxies are receding away from us was first demonstrated by Edwin Hubble in 1929, building upon the work of Henrietta Leavitt and others. Since then we’ve developed a variety of ways to measure the rate of cosmic expansion, and while they are broadly in agreement, there are small discrepancies between them. As a result we still don’t know exactly how fast the Universe is expanding, as astrophysicist Ethan Siegel has so clearly explained. Now a new method of measuring cosmic expansion may settle the issue, but it also raises more questions.

It all comes down to a physical parameter known as the Hubble constant. The bigger Hubble constant, the greater the rate of cosmic expansion. The value of the constant also tells us the age of the Universe. If you trace the expansion backwards through time, you reach the point where the Universe was extremely hot and dense, commonly known as the big bang.

Hubble’s original measurement of the constant compared the distances of galaxies with the redshift of their light. He calculated galactic distances by measuring the brightness of variable stars known as Cepheid variables, and combined them with measurements of galactic redshifts made by Vesto Slipher. He found that more distant galaxies had greater redshifts, and presumably were receding from us at a greater rate. Hubble’s original value for the constant was about 500 km/s/Mpc, which caused a bit of a cosmological crisis. If the value was correct, the Universe was only about 2 billion years old, which contradicted geological evidence that showed the Earth was more than 4 billion years old.

Credits: NASA, ESA, A. Feild (STScI), and A. Riess (STScI/JHU)

Over time our measurements of the Hubble constant got better, and the results settled around a much smaller value of around 70 km/s/Mpc, putting the age of the Universe at about 14 billion years. We also developed different ways to calculate the Hubble constant using different types of data, and they each produced similar results. This means we know for sure that the Universe is expanding, and we have a pretty good handle on just how fast it’s expanding. But while these different methods broadly agreed, they didn’t exactly agree. It was generally thought that as our measurements got better this discrepancy would go away, but it didn’t. Something was clearly wrong with our understanding of cosmic expansion.

Modern measurement tensions from the distance ladder (red) with CMB (green) and BAO (blue) data. Image credit:
“Cosmological implications of baryon acoustic oscillation measurements”, Aubourg, Éric et al. Phys.Rev. D92 (2015) no.12, 123516.

Hubble’s method of comparing distance with redshift has been extended by shifting from Cepheid variables to supernovae. A particular type of supernova known as Type IA allows us to determine galactic distances across billions of light years. In 2016, observations from the Hubble telescope using this approach gave a value of 73.24±1.74 km/s/Mpc, which is on the high side of modern values.

A different approach looks at fluctuations in the cosmic microwave background (CMB). The CMB is the thermal remnant of the big bang, and while it is quite uniform in temperature, there are small-scale fluctuations. As the Universe has expanded, these fluctuations are stretched, so the scale at which fluctuations peak gives a value of the rate of cosmic expansion, and thus the Hubble constant. The most precise CMB measurement of the Hubble constant was made by the Planck satellite, and gave a result of 66.93±0.62 km/s/Mpc, which is on the low side. The Planck result agrees with another method known as baryon acoustic oscillation (BAO), which looks at how galaxies cluster together across billions of light years, which gives a value of 67.6±0.7 km/s/Mpc.

Temperature fluctuations of the CMB vary at different scales. Credit: NASA/WMAP

These disagreements are problematic because they point to problems in our cosmological model. Although each result is quite sophisticated, they depend upon certain assumptions about the Universe. Our current model, known as the LCDM model, includes regular matter, dark matter and dark energy, as well as things such as how flat the Universe is on small scales. Each of these can be measured by independent experiments, but the results all have a bit of uncertainty. Tweak the values of these parameters within the model, and the value of the measured Hubble constant shifts. So we could tweak the model to make Hubble constant results fit, but tweaking models to fit your expectations is bad science.

Now there’s a new method for determining the Hubble constant, and its result is very interesting.

Diagram showing how distant light can be gravitationally lensed. ALMA (ESO/NRAO/NAOJ), L. Calçada (ESO), Y. Hezaveh et al.

Rather than looking at the CMB or measuring galactic distances, the new approach looks at an effect known as gravitational lensing. As light passes near a large mass such as a star or galaxy, it is gravitationally deflected. As a result, light from a distant object such as a quasar can be deflected around a less distant galaxy. Instead of seeing one image of the distant quasar, we see multiple images. But if we look at these lensed images things get very interesting. Each image of the quasar has taken a different path, and those paths can have different lengths. So some images reach us sooner than others. We’ve seen this effect with distant supernovae, for example, allowing us to see multiple “instant replays” of a supernova over the course of a few decades. Quasars can fluctuate in brightness, which allows us to measure the timing between lensed images of a particular quasar.

In this new approach, the team looked at several lensed quasars, and measured the timing differences. These timing differences are affected by the Hubble constant, so by measuring different lensed quasars the team could get a value for the Hubble constant. The key here is that while the results depend strongly on the value of the Hubble constant, they aren’t affected very much by other model parameters such as the amount of regular matter and dark matter. It’s a more direct measurement, and therefore less dependent on model assumptions. The result they got was 71.9±2.7 km/s/Mpc.

This agrees pretty well with the Hubble results, but not with the CMB results. Since the result is less model dependent, it raises questions about our cosmological model. Why are the CMB and BAO results so much lower than the others? It isn’t clear at this point, and while this new result is great, it doesn’t solve the mystery of Hubble’s constant.

Paper: V. Bonvin, et al. H0LiCOW V. New COSMOGRAIL time delays of HE0435-1223: H0 to 3.8% precision from strong lensing in a flat ΛCDM model. arXiv:1607.01790 [astro-ph.CO] (2017)

The post Bigger, Stronger, Faster appeared first on One Universe at a Time.

When Edwin Hubble first demonstrated the Universe was expanding in 1929, you could do a simple calculation to determine the age of the Universe. Take the rate at which galaxies expand from each other (known as the Hubble constant H) and set it equal to the inverse age of the cosmos (1/t). This simple model assumed that the Universe expands at a constant rate, thus Ht = 1. When this was first proposed within the context of the big bang model, it actually raised a few questions. Early measurements of the Hubble constant were much higher than the current accepted value, which gave a cosmic age that was actually younger than some stars. 

We now know the Universe hasn’t expanded at a constant rate. The rate of cosmic expansion is determined both by dark energy driving galaxies apart, and the overall density of matter in the Universe, which tries to slow the rate of expansion. In the early universe, matter dominated, so the rate of expansion was actually decreasing. About 6.5 billion years ago the average density of the Universe dropped to the point that dark energy began to dominate, and the Universe began expanding at an ever increasing rate. An accurate determination of the age of the Universe has to account for the initial inflationary period, then deceleration, then acceleration. If you do that you get an age of about 13.8 billion years, which is the currently accepted age.

Because of this variation in cosmic expansion, the Hubble constant has changed over cosmic time. This is why you can’t simply set Ht = 1. And yet, if you take the current Hubble constant and multiply it by the currently accepted age of the Universe, you get exactly 1 (to within known uncertainties). In other words, if the Universe had expanded at a constant rate, it would be exactly the same size and age as the Universe currently is. This is known as the synchronicity problem. It’s not a problem, per se, but rather an interesting coincidence. This hasn’t been true for any other epoch of the cosmos. It’s also not the only odd coincidence. The vacuum energy density (as determined by the Hubble constant) and the matter energy density are also about equal, and is known as the coincidence problem.

As the Universe expands the matter density drops, while the vacuum density doesn’t,  so it’s tempting to think that the synchronicity problem and the coincidence problem are two sides of the same coin. But a recent work shows this isn’t the case. By varying the parameters of a hypothetical universe, one could create a model where one is true but the other is not. These two unusual correlations are independent of each other. This raises the question of whether the two actually are related by some unknown physical process. We always have to be a bit careful with these kinds of questions. It is perfectly possible that the two “problems” are just due to random flukes. But when you start seeing coincidences in your data it is sometimes worth exploring.

If there is a connection, it will only be a matter of time before we find it.

Paper: Arturo Avelino and Robert P. Kirshner. The dimensionless age of the Universe: a riddle for our time. The Astrophysical Journal, Volume 828, Number 1 (2016) arXiv:1607.00002 [astro-ph.CO]

The post The Constant Of Time appeared first on One Universe at a Time.

The Universe is expanding. In the standard model of cosmology the rate of that expansion is given by the Hubble parameter, which is a measure of the dark energy that drives cosmic expansion. New observations of distant galaxies yield a higher than expected Hubble value. That may mean the Universe is expanding faster than we thought, but there’s no need to start rewriting textbooks just yet. 

Since the Hubble parameter measures the rate of cosmic expansion, one way to determine it is to compare the redshift of light from distant galaxies with their distance. The cosmological redshift of a galaxy is easy to measure, and is due to the fact that cosmic expansion stretches the wavelength of light as it travels across millions or billions of light years, making it appear more red. By comparing the redshifts for galaxies of different distances we can determine just how fast the Universe is expanding.

Unfortunately distance is difficult to determine. It relies upon a range of methods that vary depending on distance, known as the cosmic distance ladder. For close stars we can use parallax, which is an apparent shift of stars relative to more distant objects due to the Earth’s motion around the Sun. The greater a star’s distance the smaller its parallax, so the method is only good to about 1,600 light years. For larger distances we can look at variable stars such as Cepheid variables. We know the distance to some Cepheid variables from their parallax, so we can determine their actual brightness (absolute magnitude). From this we’ve found that the rate at which a Cepheid variable changes in brightness correlates with its overall brightness. This relation means we can determine the absolute brightness of Cepheid variables greater than 1,600 light years away. If we compare that to their apparent brightness we can calculate their distance. By observing Cepheids in various galaxies we can determine galactic distances. We can observe Cepheids out to about 50 million light years, at which point they’re simply too faint to currently observe.

The bright spot in the lower left is a supernova in the NGC 4526 galaxy. Credit: NASA, ESA, The Hubble Key Project Team, and The High-Z Supernova Search Team

Enter the supernova. In a single burst of light a supernova can outshine an entire galaxy, so they can be detected across billions of light years. While there are several types of supernovae, one type (Type Ia) has a fairly consistent maximum brightness. We know this by observing several in galaxies where Cepheids have been used to determine their distance. Just like Cepheids, we can compare the actual brightness of a Type Ia supernova with its apparent brightness and determine the distance of a galaxy.

The achilles heel of the cosmological distance ladder is that it relies upon a chain of data. The distance for supernovas depends upon the calculated distance of Cepheid variables, which in turn depend upon parallax distance measurements. With ever increasing distance comes greater uncertainty in the results. So you want your uncertainties at each step to be as small as possible, which is where this new work comes in. Using data from the Hubble Space Telescope’s Wide Field Camera 3, a team measured about 2,400 Cepheid variables in 11 galaxies where a Type Ia supernova had also occurred. This allowed them to reduce the uncertainty of supernova distance measurements. They then compared the distances and redshifts for 300 supernovae to get a measure of the Hubble parameter accurate to within 2.4%.

That by itself is good work, but the result was surprising. The value for the Hubble parameter they got was about 73 km/s per megaparsec, which is higher than the “accepted” value of 69.3. The difference is large enough that it falls outside the uncertainty range of the accepted value. If the result is right, then it means the Universe is expanding at a faster rate than we thought. It could also point to an additional dark energy component in the early Universe, meaning that dark energy is very different than we’ve supposed.

But we shouldn’t consider this result definitive just yet. The use of supernovae to measure the Hubble parameter isn’t the only method we have. We can also look at the way galaxies cluster on large scales, and fluctuations in the cosmic microwave background. Each of these gives a slightly different value for the Hubble parameter, and the “accepted” value is a kind of weighted average of all measurements. The variation of values from different methods is known as tension in the cosmological model, and any new claim about dark energy and cosmic expansion will need to address this tension. If the supernova method is right and the Universe really is expanding faster than we thought, why do other methods yield a value significantly smaller than the true value? It could be that there is some bias in one or both of the methods that we haven’t accounted for. Planck, for example, has to account for gas and dust between us and the cosmic background, and that may be skewing the results. It could be that the supernovae we use as standard candles to measure galactic distance aren’t as standard as we think. It could also be that our cosmological model isn’t quite right. The current model presumes that the universe is flat, and that cosmic expansion is driven by a cosmological constant. We have measurements to support those assumptions, but if they are slightly wrong that could account for the differences as well.

This new result does raise interesting questions, and it confirms that the discrepancy between different methods is very real. Whether that leads to a new understanding of cosmic expansion and dark energy is yet to be seen.

Paper: Adam G. Riess, et al. A 2.4% Determination of the Local Value of the Hubble Constant. arXiv:1604.01424 [astro-ph.CO] (2016)

The post New Evidence Challenges Rate Of Cosmic Expansion appeared first on One Universe at a Time.

With all the news about BICEP2 and the possible detection of early inflation, there have been a lot of misconceptions about what inflation actually is.  One of the biggest is the idea that during inflation the universe expanded faster than light.  It’s a misconception that even many experts get wrong, and is so common that there’s a technical arxiv paper addressing these misconceptions.  It’s easy to see how this misconception arises.  After all, during inflation, two atoms a meter apart just before would find themselves about a light year apart within a fraction of a second.  How is that not moving faster than light?  It all has to do with the subtlety of general relativity.

Before we talk about inflation, let’s talk about a similar effect we see in the universe today, known as cosmic expansion.  When we look at distant galaxies, we see that the light of more distant galaxies is redshifted more than closer galaxies.  Now we know that light can be redshifted when objects move away from us, known as the Doppler effect.  For this reason, this effect is often described as galaxies moving away from us.  But redshift can also occur due to the expansion of space itself.  That is, space expands, but the galaxies aren’t moving through space.  The first is a property of special relativity, and is due to the motion of objects through space. The second is a property of general relativity, and is due to an expansion of space.

Supernova magnitude-redshift observations compared to GR models and SR. Credit: Davis and Lineweaver.

Since both effects give a redshift to distant galaxies, how do we know that cosmic expansion is due to an expansion of space and not relative motion?  If the redshift were due to relative motion, then the light of distant galaxies would be redshifted when leaving the galaxy, and that means the light would also appear dimmer.  If the redshift is due to cosmic expansion, then light leaves a distant galaxy without being redshifted, and therefore also not dimmed.  Only later is the light redshifted due to expansion.  This means you can compare the brightness of distant supernovae with their redshift, known as the magnitude-redshift relation.  What we find is that the magnitude-redshift relation matches expansion extraordinarily well.  It doesn’t match the relative motion model at all.

So we know that space is actually expanding, but what does that actually mean?

Cosmic expansion is determined by what is known as the Hubble constant.  Currently our best measurement of the Hubble constant is about 20 km/s per million light years.  This means that two points in space a million light years apart are moving away from each other at 20 kilometers each second.  Since all of space is expanding, the greater the distance between two points in space, the faster they move apart.  So two points 10 million light years apart are moving away at 200 km/s, and so on.  Because of this, if you consider two points far enough apart, they will be moving away from each other faster than the speed of light.  The speed of light is about 300,000 km/s, which, given our current Hubble constant is the separation speed for two points 15 billion light years apart.

Now you might think then that a galaxy 16 billion light years away from us must be moving away from from us faster than light.  You could say that the the galaxy appears to be moving faster than light, but in actuality it is space that is expanding between us.  The galaxy itself isn’t moving much at all.  It’s not as if that distant galaxy is defying relativity.  After all, from that distant galaxy’s perspective we are moving away from it faster than the speed of light.  The key point to remember is that this is due to cosmic expansion, not galactic motion.  And cosmic expansion is not faster than light, even though very distant objects can appear to be moving faster than light.

Which brings us to inflation.  Many of the popular articles unfortunately state that during inflation the Universe was expanding faster than light, which isn’t true.  What is true is that during inflation the rate of spatial expansion was much larger.  This means that the distance at which objects appear to move apart faster than light is much smaller, but it does not mean that the Universe expanded faster than light.

We still don’t understand the mechanism that triggered inflation, but we do know that inflation doesn’t violate the speed of light.  During inflation the rate of expansion was tremendous, but even today space continues to expand, just at a much smaller rate.

Paper: Tamara M. Davis, Charles H. Lineweaver. Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe. arXiv:astro-ph/0310808 (2003).

The post Traveling Without Moving 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.

Yesterday I wrote about a galaxy cluster formed by the collision of four clusters.  This raised a question by a reader who wondered how galaxies can collide if they are all moving away from each other.  The short answer is that they aren’t all moving away from each other, but if the universe is expanding, shouldn’t all the galaxies be racing away from each other?  It all has to do with the way cosmic expansion and gravity work, and it tells us something about the nature of dark matter and dark energy.If the universe were not expanding, then you would expect some galaxies to collide with each other.  The gravitational attraction between galaxies would cause them to accelerate slowly toward each other, eventually leading to their collision.  Some galaxies might be moving to fast to be caught in the gravity of another galaxy, but even then there could be collisions.  In a static universe we would expect galaxies to be moving in all directions, with galactic collisions scattered throughout the visible universe.

Diagram from Hubble’s 1929 paper. Credit: Edwin Hubble.

But in 1929 Edwin Hubble published a paper showing the universe was not static.  He compared the distances of several galaxies with their redshift, and found that their motion followed a relation where the more distant the galaxy, the greater its redshift.  This meant galaxies were not simply moving at random, as you would expect in a stable, uniform universe. Instead, the more distant the galaxy the faster it is moving away from us (thus the greater its redshift). This relation between distance and speed is the same in all directions, which means the universe seems to be expanding in all directions.

If these galaxies were all expanding away from a single point (a common misconception about the big bang), then you wouldn’t expect to see many galaxies colliding with each other.  In fact, if they all radiated away from a single point, then you would expect the more distant galaxies to spread more thinly than closer galaxies.  This isn’t what we see at all.

Galaxy clusters in the universe.
Credit: Sloan Digital Sky Survey.

What we see is that galaxies tend to clump together in clusters, and those clusters tend to clump into superclusters.  Within a cluster or supercluster the galaxies tend to gravitate toward each other, leading to collisions between galaxies here and there.  Between the superclusters are large voids where there is very little.  What this tells us is that the galaxies aren’t just racing from a single point in space.  Instead the entire universe is expanding.  It is cosmic expansion that gives the relation between distance and redshift.  But even distant galaxies can be moving slowly relative to each other, so they can clump together just as easily as nearby galaxies.  So galaxies throughout the universe tend to clump into clusters and superclusters.

What’s particularly interesting is that if you look at the distribution of galaxies in the universe, you find that they clump in a particular way.  If the galaxies were just made of matter, then they wouldn’t clump as much as they do.  So the level of clumping means there must also be dark matter.  The gaps between the galaxies are larger than what you would expect for matter and dark matter alone, so there must be something accelerating the expansion of the universe, which is dark energy.

So rather than galaxy collisions being a mystery, they are actually a part of the obsevational evidence telling us that dark matter and dark energy exist.

The post Push Me, Pull You appeared first on One Universe at a Time.

Last time I discussed how light can be red shifted or blue shifted by the relative motion of a light source, an effect known as the Doppler effect.  I also outlined how gravity can also shift light to the red or blue.  There is one other way light can be red shifted, and that is due to the expansion of the universe.

One evidence for cosmic expansion comes from Hubble’s law, which demonstrates that the more distant a galaxy the more redshifted its light tends to be.  This relationship between distance and redshift is roughly linear, and it is typically explained by imagining galaxies drawn on an inflating balloon.  As the balloon expands, galaxies near each other move slightly apart, while widely separated galaxies separate much more quickly.

This tends to reinforce the idea that Hubble redshift is due entirely to the relative motions of galaxies, that it is caused purely by the standard Doppler effect.  In actuality it’s a little more complicated.  There is a redshift due to the relative motion of a galaxy when the light leaves the source, but light can travel for billions of years before reaching, and during that time the universe continues to expand. Since space itself is expanding, the wavelength of the traveling light also stretches.  This means that while the light travels, it continues to redshift due to cosmic expansion.  You can see a demonstration of this in the video below.

Of course one could ask the question:  what if this redshift isn’t caused by expansion?  What if light just spontaneously redshifts over time.  This idea is known as the “tired light” model, and if it were true the correlation between redshift and distance would be the same.  However we know that isn’t what’s happening because the light from distance galaxies is also time dilated.  In other words, when we watch a supernova occur in a distant galaxy, it happens in slow motion from what we would expect. This slowdown is caused by the relative motion of the galaxy, since the faster something moves relative to us the more its time appears to slow down.

When we look at the time dilation effect for galaxies as a function of distance we see that it agrees with the expanding universe model and not tired light.  This is also how we know the visible universe has a radius of about 46 billion light years even though the universe is only about 13.7 billion years old.  The light from the most distant galaxies was about 13 billion years from us when the universe was young, but during the light’s 13 billion year journey the universe has continued to expand.

The post Bend and Stretch appeared first on One Universe at a Time.

Time for more on the cosmic microwave background (CMB).  This time for something really cool.  You’ll remember that the CMB isn’t perfectly uniform but instead has little variations in temperature.  Some regions are slightly hotter, and some are slightly cooler.  This is similar to having ripples on the surface of a pond, where some parts of the surface are slightly higher than the average pond-level, and some slightly lower.

The pond analogy is a pretty good one, because you could imagine studying the ripples to figure out what caused them.  If you could show, for example, that ripples moved out in circles from a central point, you would know that a stone was dropped in the water there.  If you measure the size of the largest ripples, and know their speed, you could calculate when the stone was dropped.  You could look at other ripples and figure out they were caused by the wind, and maybe a large ripple caused by a passing boat.

We can do a similar thing with the CMB.  The way we do it is by looking at the power of the CMB at different scales, called a power spectrum.   In mathematical terms we expand the ripples into a sum of multipole moments. Basically just take the total average temperature of the whole sky, then split the sky into two regions and take an average of each section, then split and average again, and so on.  Keep doing that, and you get an “average” temperature at each scale, which you can see in the figure below.

Now some scales are more powerful than others, and that tells us about what factors are causing the ripples.  Using our pond analogy you might have more little ripples on the pond than big ones, which would tell you that more stones cause the ripples than boats.  For the CMB, the effects are things like the value of the Hubble constant, how much dark energy and dark matter you have, etc.

In the figure below, I’ve not only plotted the observed power spectrum as data points, I’ve also plotted a theoretical curve that agrees with the data.  This theoretical curve is the shape you’d expect for a universe that is 74% dark energy, 22% dark matter, 4% regular matter (stars, planets and us), and is about 13.7 billion years old.  We have other ways of determining each of these values, and they agree with these values.

The amazing thing is that all these values fit in this single curve.  If the values were different the peaks would shift left or right, or be higher or lower.  While the image of the CMB is wonderful, with its swirls of color, this graph is even more wonderful.  It tells us that our understanding of the universe is on track.

Not bad for looking at ripples in our cosmic pond.

The post Ripples on the Cosmic Pond appeared first on One Universe at a Time.

A while back I wrote about the relationship between the distance of a galaxy from us and the speed at which it moves away from us.  This relationship is now known as Hubble’s law, after Edwin Hubble, who first plotted the speed of a couple dozen galaxies versus their distance in 1929.  What he found was a linear relationship between speed and distance.  In other words, it seemed the speed of a galaxy divided by its distance was a constant, now known as the Hubble constant.  It was the first solid evidence of an expanding universe.

Since Hubble’s day we’ve been able to measure the speeds and distances of more than four thousand galaxies, so we’re able to get a much better measurement of the Hubble constant.  All this extra data has made things very interesting.

For one thing, as we measure more distant galaxies we have to change Hubble’s law a bit.  Hubble’s original relation was between speed and distance, but at really large distances galaxies are moving away from us at a large fraction of the speed of light.  This means we have to take special relativity into account at large distances.  For this reason the Hubble relation is now expressed not in terms of galactic speed, but in terms of a measure of redshift known as z.  The nice thing about z is that it allows special relativity to be basically factored out of observations.  If Hubble’s model holds, then a plot of z versus distance should be a straight line.

A wonderful aspect of observational astronomy is that when you look at more and more distant objects, you are also looking further back into time.  If a galaxy is a billion light years away from us, the light we observe left that galaxy a billion years ago.  This means galactic distance is a also a measure of the past.  As a result, the Hubble constant is not just a measure z versus distance, but also a measure of z versus time.  In other words it tells us the speed at which the universe is expanding.

Redshift vs distance.

In the past decade, however, we’ve found that Hubble’s constant doesn’t quite hold, as you can see in the figure above.  The linear relation is plotted as a black line, but the best fit to the data is the red line.  The red line isn’t straight, but instead curves slightly upward. This means at really large distances the redshift is greater than we would expect.  The expansion speed of the universe is not constant, but is getting larger.  In other words, the universe is accelerating.

Of course this means something is likely causing this acceleration. That something is known as dark energy.

The post Cosmic Energy appeared first on One Universe at a Time.

One of the more interesting astrophysical discoveries of the 20th century is the fact that the universe is expanding. The result was so unexpected that even Einstein discarded its prediction within general relativity. Einstein went so far as to introduce an extra constant in his equations specifically to prevent an expanding universe model. He would later call it his greatest blunder.

But how do we know the universe is actually expanding? For this we need to use the handy-dandy Doppler effect. You might remember that the observed color of light can be effected by the relative motion of its source. If a light source is moving toward us, the light we see is more bluish than we would expect (blue shifted). If a light source is moving away from us, the light is more reddish (red shifted). The faster the source is moving, the greater the shift.

We have measured this color shift for lots of stars, galaxies and clusters. We’ve also determined their distances (exactly how will be a post for another day). If we plot a graph of the distance of galaxies and clusters versus their redshift we find something very interesting. I’ve plotted such a graph below, and you can see there is almost a linear relationship between distance and redshift.

Distance vs speed for galaxies.

This means galaxies are not simply moving at random, as you would expect in a stable, uniform universe. Instead, the more distant the galaxy the faster it is moving away from us. This relation between distance and speed is the same in all directions, which means the universe seems to be expanding in all directions.

Since this relationship is linear, you can fit this data to a line. The slope of the line is known as the Hubble constant, named after Edwin Hubble, who was one of the first to observe this relationship. When I did a simple linear fit to the data (the dashed line), I got a Hubble constant of 68.79 km/s per megaparsec. This is in the range of the accepted value.

Of course if the universe is expanding, then it must have been smaller in the past. If we assume the universe expands at a constant rate, then we can trace its size back in time to a point where the universe would have zero volume. In other words, the universe has a finite age, and it began very small, very dense (and therefore very hot). We call that starting point the big bang. If you do the math, the age of the universe is simply the inverse of the Hubble constant. Given our value, this puts the age of the universe at about 14.5 billion years. More accurate calculations put the age at 13.75 billion years.

The post Hubble’s Constant appeared first on One Universe at a Time.