Our universe is expanding. We’ve known this for nearly a century, and modern observations continue to support this. Not only is our universe expanding, it is doing so at an ever increasing rate. But the question remains as to what drives this cosmic expansion. The most popular answer is what we call dark energy. But do we need dark energy to account for an expanding universe? Perhaps not. 

The idea of dark energy comes from a property of general relativity known as the cosmological constant. The basic idea of general relativity is that the presence of matter warps spacetime. As a result, light and matter are deflected from simple straight paths in a way that resembles a gravitational force. The simplest mathematical model in relativity just describes this connection between matter and curvature, but it turns out that the equations also allow for an extra parameter, the cosmological constant, that can give space an overall rate of expansion. The cosmological constant perfectly describes the observed properties of dark energy, and it arises naturally in general relativity, so it’s a reasonable model to adopt.

In classical relativity, the presence of a cosmological constant simply means that cosmic expansion is just a property of spacetime. But our universe is also governed by the quantum theory, and the quantum world doesn’t play well with the cosmological constant. One solution to this issue is that quantum vacuum energy might be driving cosmic expansion, but in quantum theory vacuum fluctuations would probably make the cosmological constant far larger than what we observe, so it isn’t a very satisfactory answer.

Despite the unexplainable weirdness of dark energy, it matches observations so well that it has become part of the concordance model for cosmology, also known as the ΛCDM model. Here the Λ is the symbol for dark energy, and CDM stands for Cold Dark Matter. In this model there is a simple way to describe the overall shape of the cosmos, known as the Friedmann–Lemaître–Robertson–Walker (FLRW) metric. The only catch is that this assumes matter is distributed evenly throughout the universe. In the real universe matter is clumped together into clusters of galaxies, so the FLRW metric is only an approximation to the real shape of the universe. Since dark energy makes up about 70% of the mass/energy of the universe, the FLRW metric is generally thought to be a good approximation. But what if it isn’t?

A new paper argues just that. Since matter clumps together, space would be more highly curved in those regions. In the large voids between the clusters of galaxies, there would be less space curvature. Relative to the clustered regions, the voids would appear to be expanding similar to the appearance of dark energy. Using this idea the team ran computer simulations of a universe using this cluster effect rather than dark energy. They found that the overall structure evolved similar to dark energy models. That would seem to support the idea that dark energy might be an effect of clustered galaxies.

It’s an interesting idea, but there are reasons to be skeptical. While such clustering can have some effect on cosmic expansion, it wouldn’t be nearly as strong as we observe. While this particular model seems to explain the scale at which the clustering of galaxies occur, it doesn’t explain other effects, such as observations of distant supernovae which strongly support dark energy. Personally, I don’t find this new model very convincing, but I think ideas like this are certainly worth exploring. If the model can be further refined, it could be worth another look.

Paper: Gabor Rácz, et al. Concordance cosmology without dark energy. Monthly Notices of the Royal Astronomical Society: Letters DOI: 10.1093/mnrasl/slx026 (2017)

The post Who Needs Dark Energy? appeared first on One Universe at a Time.

The evidence for dark energy lies in our ability to relate the redshift of a galaxy with it’s distance. While we often talk about how the observed redshift of a galaxy allows us to determine its distance, that assumes our understanding of dark energy is correct. To prove dark energy is real we have to measure redshift and distance independently, and that takes a bit of doing.

Measuring redshift is fairly straightforward. By comparing the spectrum of a distant galaxy with the known spectra of atoms and molecules here on Earth, we can determine the amount of redshift expressed in a quantity known as z. To measure distance, however, we need to use observations of a kind of supernova known as type Ia. These are often described as “standard candles” that always explode with the same brightness, but that isn’t actually the case. Some type Ia supernovae are brighter than others, so you can’t simply use their observed brightness as a measure.

Raw light curves (top) vs. calibrated light curves (bottom) for type Ia supernovae.

Type Ia supernovae are identified by their emission spectrum. Their spectrum lacks hydrogen lines, and has a distinct silicon emission line when it is near maximum brightness. From this we can clearly distinguish type Ia from other supernovae. What we know from observing type Ia supernovae in nearby galaxies is that there is a specific relation between their peak brightness and the time it takes for them to decay. Bright supernovae shine longer than dim supernovae. From the ratio of peak to width of their light curve, we can calibrate these supernovae to determine their absolute magnitude. Comparing that with their observed magnitude we can determine their distance.

Calculating distance is based upon two assumptions. The first is that our view of the supernovae is relatively unobscured. We calculate distance using the inverse-square relation for light, but that only works if there isn’t gas or dust absorbing some of the light. While there can be gas and dust between us and a supernova, it wouldn’t absorb all frequencies of light by the same amount. Blue wavelengths are absorbed much more than red wavelengths (creating an effect known as reddening) and infrared wavelengths aren’t absorbed much at all. Since distant galaxies are deeply reddened, gas and dust have little effect on their observed brightness. So we know our first assumption is valid.

The second assumption is that nearby type Ia supernovae are the same as distant ones. Interestingly, in recent years there’s been some evidence that might not be the case. Recent observations of a large number of supernovae seem to show two classes of type Ia supernovae, with slightly different ratios. If this is true, then it could readjust the amount of dark energy the universe has. However this would be a minor adjustment to our understanding of cosmology, not a revolutionary change. While supernovae are a great way to observe the effects of dark energy, they aren’t the only way. We can also look at things such as the clustering of galaxies on large scales, and the fluctuations within the cosmic microwave background to determine the amount of dark energy in the universe. What we find is that they all agree reasonably well.

So while type Ia supernovae aren’t standard candles, they are standardizable candles, and they tell us a great deal about the cosmos.

Paper: Peter A. Milne et al. The Changing Fractions of Type Ia Supernova NUV–Optical Subclasses with Redshift. ApJ 803 20. (2015)

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

Recently I’ve been asked about reports of new research showing the universe isn’t accelerating. If true, it would mean that dark energy doesn’t exist, which would be a good way to solve the mystery. While there is the occasional headline making such a claim, there isn’t a great deal of evidence to support the idea. There is, however, plenty of evidence that dark energy exists.

The most recent paper claiming to eliminate (or at least weaken) dark energy showed up recently on the arxiv. It focuses on one keystone of dark energy evidence, the observations of distant supernovae. One particular type of supernova known as Type Ia has the useful property of exploding with a fairly uniform brightness. This means they can be used as “standard candles” to determine their distance. Basically you can observe its apparent brightness and compare it to its actual brightness to get a distance. Observation of some of the most distant supernovae at the time led to the Nobel-winning discovery of dark energy.

The no acceleration model compared to observational data.

But recently there’s been evidence that there is more variation within Type Ia supernovae than originally thought, including a dimmer variation known as Type Iax. This means the uncertainty in the actual brightness of Type Ia supernovae might be greater than we’ve been using, which is where this new paper comes in. Basically what the authors do is analyze the observations we have of distant supernovae using larger uncertainties. They then compare this data to both the accelerating and non-accelerating cosmological models. What they find is that the confidence level of the accelerating model is lowered, which is exactly what you would expect if you make your uncertainties larger. They also find that support for no acceleration increases, which is also what you’d expect with larger uncertainties.

Their conclusion is that the non-accelerating model is “still in the game” as it were, since larger uncertainties make the distinction between the two models less clear. But the evidence doesn’t support that conclusion. The strongest candidate by far is still an accelerating universe based upon this data, and dark energy is supported by other evidence such as galactic clustering and the cosmic microwave background.

In light of new supernova observations, it’s good to keep testing our cosmological models, but so far the standard LCDM model of an accelerating universe is the best model we have.

Paper: J T Nielsen, et al. Marginal evidence for cosmic acceleration from Type Ia supernovae. arXiv:1506.01354 [astro-ph.CO] (2015)

Paper: Peter A. Milne et al. The Changing Fractions of Type Ia Supernova NUV—Optical Subclasses with Redshift. ApJ 803 20 (2015)

The post So You’re Saying I’ve Got a Chance appeared first on One Universe at a Time.

While the search for dark matter particles often hits the news, there are also efforts underway to detect dark energy particles. As with dark matter, the experiments thus far have largely determined what dark matter isn’t rather than what it is. 

There are two basic ways to account for dark energy. One is known as the cosmological constant. In this model, dark energy is an inherent aspect of the structure of space and time. Thus, throughout the universe there is a constant, uniform expansion of spacetime that gives the effect of dark energy. This model is the simplest way to account for dark energy, as it’s just a matter of adding a term to the usual general relativity equations. It also agrees with observations so far. But simply adding a term to your equations seems like a bit of a tweak model. General relativity doesn’t require a cosmological constant, it just allows for one. There’s no reason why there should be such a constant other than the fact that it fits observation. So lots of alternatives have been proposed.

The most popular type of alternative is to propose some type of scalar field. The idea is that the universe would be filled with a scalar field that results in dark energy. That may seem even more crazy than a cosmological constant, but the Higgs boson is a result of a scalar Higgs field introduced to account for particle mass, and we’ve actually detected it. There are several variations of the scalar field idea, but most of them can’t be tested using current data. But one version known as chameleon fields has just been tested, and failed the test.

The basic experiment. Credit: Paul Hamilton, et al.

The chameleon field is a “fifth force” field that interacts with itself  to produces the effects of dark energy in deep space, but also gets inhibited by the presence of mass. In this way you get cosmic expansion between galaxies, but you don’t see its effect in galaxies (or in our solar system). Since the presence of mass makes it “hidden,” it acts as a kind of cosmic chameleon, hence the name. Normally I wouldn’t put much credence in a “just so” model like this, but a few months ago it was demonstrated that the model could actually be tested. Because of its chameleon effect, the field could be “trapped” within a vacuum cavity. By making the matter in the chamber as little as possible, the chameleon field would strengthen in the chamber. As a result, the effective gravitational force within the vacuum is altered. Using an atom interferometer (basically a double-slit experiment using atoms instead of electrons) the change in gravity could be measured. What the team found was that there was no measured effect to the limits of their experiment.

This basically rules out the chameleon field and similar models. There’s still a few ways the model could be tweaked to still exist within the limits of this experiment, but it doesn’t look good for chameleon fields. That’s not particularly surprising, since most proposed models will be wrong. What makes this interesting is that we’re now actually testing dark energy models in the lab.

Paper: Paul Hamilton, et al. Atom-interferometry constraints on dark energy. arXiv:1502.03888 [physics.atom-ph] (2015)

The post Karma Chameleon appeared first on One Universe at a Time.

We know the universe is expanding, and we know it is doing so at an ever increasing rate. This cosmic acceleration is part of the evidence for dark energy, which current observations put at about 68% of the observable universe. But beyond its existence as some kind of energy, we’re still trying to determine just what dark energy is.

The most widely accepted model for the universe, sometimes called the standard model of cosmology, is known as the ΛCDM model. The Λ or “lambda” refers to the dark energy parameter known as the cosmological constant (which often uses lambda to represent its value). The CDM stands for Cold Dark Matter, which is the type of dark matter currently best supported by observation. In the ΛCDM model, the matter of the universe consists of regular matter (about 5%), cold dark matter (about 27%). Dark energy is then caused by a cosmological constant, which is a property of space and time itself, giving rise to dark energy.

The ΛCDM model makes a couple of very important predictions about the universe that can be tested against observation. The first is the the universe is, on average, flat. This means that local masses can curve space around them (with the effect of gravitational attraction), but since the cosmological constant curves space in an opposite way (cosmic expansion), the total average should be zero. Current observations agree with this prediction, though is a hint in the data of the cosmic microwave background that the curvature might not be exactly zero, as I’ve written about before.

Predictions of different cosmological models.
Credit: NASA, CXC, M. Weiss

A second prediction is about the equation of state for dark energy. This boils down to a parameter in cosmology known as w. According the the ΛCDM model, w should be exactly equal -1. If w = -1, then dark energy is a constant, and can be described in general relativity by the cosmological constant. Again, observations agree pretty well with this prediction, but a new paper summarizing observational data from Pan-STARRS seems to show that w might not be -1 after all.

Pan-STARRS (the Panoramic Survey Telescope & Rapid Response System) has been making a medium deep sky survey that has observed 112 type 1a supernovae. These supernovae observations were combined with other observations for a total of 313 type 1a supernovae. Type 1a supernovae are useful because they have a consistent maximum brightness. This mean you can observe their apparent brightness to determine how far away they are. You can also measure the redshift of their light to determine cosmic expansion. So observations of these supernovae let us determine the value of w.

What the team found was that if you just take the supernovae data, the ΛCDM model gives an experimental value for w of -1.015 give or take about 0.1 either way, which agrees with the model. But if you take the supernova data and combine it with other observational data, including the Planck observations of the Cosmic Microwave Background and observation of the Baryon Acoustic Oscillation, then you get a different value, w = -1.186 give or take 0.07. If this value is correct then the ΛCDM model isn’t an accurate description of the universe.

If the w parameter is truly less than one, then inflation cannot be due to a cosmological constant. One of the more popular models giving a w value less than -1 is known as phantom energy. This would be a particularly strong form of dark energy, and if true could mean the universe would continue to expand at an ever accelerating rate until it ended as a Big Rip.

Before you run out and tell your friends that the standard cosmological model is wrong, keep in mind that these results are tentative. They also aren’t overly strong. In any observational result there is a chance for observation bias, which would skew the results a bit. Then there is the fact that there can be random variations in the data. The reliability of an observation is often given in a statistical term known as sigma. In this case the result is at 2-sigma, which means there is a 1 in 20 chance that the deviation from w = -1 is just random variation. In science we generally set the bar at 5-sigma, which is about a 1 in 1.7 million chance of being random variation. The authors themselves note that the data isn’t strong enough to make a clear determination.

Future observations by Pan-STARRS will likely triple the number of supernovae we can use to determine w. So in time the result, whether it agrees or disagrees with ΛCDM, will reach a strong confidence level. By then we’ll know if our standard cosmological model is correct, or if it faces a phantom menace.

The post The Phantom Menace appeared first on One Universe at a Time.

Imagine a stadium filled with people. With everyone is in their seats, waiting for the game to begin, there is an undercurrent of noise. A few words between friends, the scuffle of shoes, the creak of a chair. All of these little sounds fill the stadium with a background of white noise.

Sound is a vibration in air. As objects vibrate, they create variations in air pressure, and these variations then propagate through the air as waves of sound. If we could see these sound waves, they would look like ripples moving through air molecules. These ripples cause the air molecules to be bunched together a bit in some regions (higher pressure), and spread apart more in others (lower pressure). All the sounds create these ripples, and all these sound ripples overlap. As a result, all the small noises in the stadium create a seemingly random variation of clustered air molecules.

But these variations are not entirely random. Since each sound wave spreads outward at the speed of sound (about 330 m/s), the size of these ripples are all very similar. If speed of sound were faster, the ripples would be more spread out, and if the speed of sound were slower they would be closer together. Thus, the scale of sound ripples in the stadium is determined by the speed of sound. If we could see the ripples in the stadium, we could use the scale of these ripples to deduce the speed of sound in the stadium.

In astrophysics a method similar to this is used, and it is extremely important to our understanding of dark energy. It is known as baryon acoustic oscillation (BAO).

From our position in the universe, we see a vast sea of galaxies in every direction, with the glow of the cosmic microwave background coming from the distant edge of the visible universe. This is not because the universe is an enclosed bubble, but because the more distant the source of light, the younger the universe was when the light began its journey. Still, this view is similar to the stadium, with the galaxies like air molecules and the stadium seats marking the oldest and most distant region of the visible universe.

Just as the spectators in the stadium create a background of noise, the early universe was filled with acoustic fluctuations. As the universe expanded, those variations worked to clump some matter together, while pushing away other matter. As the galaxies formed, the effect of those early ripples can be seen in the clumping of galaxies, just as the sound ripples produce a clumping of air molecules.

Distribution of galaxies in our universe. Credit: SDSS

Just as the scale of sound ripples is determined by the speed of sound, the scale of the cosmic ripples is determined by the rate of cosmic expansion. This means we can look at the scale at which galaxies clump together and determine the rate at which the universe has expanded. If the universe had expanded at a greater rate, the clumps would be more spread apart, and if the universe had expanded more slowly, the clumps would be closer together. This is important for our understanding of dark energy because it provides another way to determine just how much dark energy there is in the universe.

The first evidence for dark energy came from the comparison of the distance of galaxies with their redshift (a measure of how fast they move away from us). We found that the universe is not only expanding, but is accelerating due to dark energy. Another dark energy measurement can be seen in variations of the cosmic microwave background temperature. These variations put dark energy at about 68.3% of the universe.

With the baryon acoustic oscillations, we now have another way to accurately determine the amount of dark energy. Here we can use the redshifts of galaxies, and determine the scale at which galaxies tend to clump. For an accurate measurement we must make as large a survey of galaxies as possible, stretching out billions of light years. At present the largest such sky survey is the Sloan Digital Sky Survey (SDSS), the result of which can be seen in the figure above. From this survey, the present scale for clumping (known as the standard ruler) is about 490 million light years. This puts dark energy at about 69.2% of the universe.

Even though we don’t fully understand dark energy, we do see its effect, and we see it in multiple ways, including its effect on the sounds of the cosmic stadium.

The post Sound It Out appeared first on One Universe at a Time.

Dark energy is perhaps the least understood aspect of modern cosmology. We first obtained evidence of its existence via the 1998 discovery that the universe is not only expanding (which we’ve long known), but that the rate of expansion is accelerating. This was done by observing the redshifts of distant supernovae, and won the discovers a Nobel prize. Since then observations of the cosmic microwave background have found that dark energy makes up about 70% of the matter-energy in the universe. This is consistent with observations of acceleration.

So we see evidence of dark energy from multiple observations, and it has a real and fundamental effect on the structure and evolution of our universe, but we have no idea what it is. There are, however, several theoretical proposals. Two of the most popular are the cosmological constant model and the scalar field model.

The cosmological constant model proposes that dark energy is a property of space and time itself. The idea was first proposed by Einstein to allow for a steady-state universe. When the universe was found to be expanding, Einstein discarded the idea, considering it his greatest blunder. The model was revived when it was demonstrated that the universe is accelerating. Instead of simply balancing the attraction of gravity, the cosmological constant acts as a repulsive force to drive galaxies apart.

The scalar field model proposes that space is permeated by some kind of energy field. The most popular field is known as quintessence, but there are other variations as well. This “dark energy” field drives the accelerating expansion of the universe.

The main difference between these two models is that the scalar field models allow for the strength of dark energy to vary in time and space. Since dark energy is a dynamic energy field, it can change and shift over time. The cosmological constant, on the other hand, requires that cosmic acceleration must be constant. Since the cosmological constant is an inherent property of space-time, it cannot vary.

Until recently, there hasn’t been enough observational data to distinguish between the two models. We know that the universe is accelerating, but we haven’t been able to determine the rate of this acceleration over time. But recently the Baryon Oscillation Spectroscopic Survey (BOSS) team has observed the effects of dark energy over the longest cosmological period thus far.

Our current universe is dominated by dark energy. The matter and dark matter of galaxies and clusters are so widely dispersed that their gravity is not strong enough to overcome the repulsion of dark energy. This is why the current universe expands at an ever increasing rate. In the early universe, however, matter was more densely packed, and its gravity was strong enough to overcome dark energy. In this early period, the universe was expanding, but the rate of expansion was decreasing. About 5 billion years ago, the population density of galaxies dropped to the point where the universe transitioned from a decelerating universe to an accelerating one. Measuring this transition period is critical to determining whether dark energy is constant.

The BOSS team observed the spectra of light from more than 48,000 distant quasars. The space between us and the quasars is not empty, but contains thin wisps of hydrogen gas, and this hydrogen absorbs some of the light from the quasars. By observing the quasar spectra, the team could determine the amount of hydrogen between us and the quasar. Since the quasars all lie at different distances from us, and in different locations in the sky, the team could create a map of the distribution of hydrogen from the early universe to now, as you can see in the figure below.

By comparing the hydrogen distribution at various times, they could determine if dark energy has distorted this distribution over time. If dark energy has varied in space and time, then the hydrogen distribution would develop clumps and voids consistent with that variation. If instead dark energy is constant, then any hydrogen clumping would be solely due to the gravitational pull of matter in the universe. These observations went as far back as 10 billion years ago, long before the transition period of the universe where the dark energy variation would be most noticeable.

What the team found was that over that period dark energy remained surprisingly constant. This doesn’t completely disprove the scalar field models, but it lends support to the cosmological constant model. This is encouraging, because the cosmological constant is more theoretically simple than the scalar field models. So it would seem that the simplest cosmological model is supported by the evidence.

Of course this is just the first step in understanding dark energy. Even if we confirm it as a cosmological constant, that still doesn’t explain how the constant has a small but non-zero value. There is also the nature of dark matter, which remains to be fully determined, and we may find a connection between the two down the line.

But for now we have solid evidence that the cosmological constant is at least a good description of dark energy. That in itself is pretty boss.

The post Like a BOSS appeared first on One Universe at a Time.

The Soudan Iron Mine in Northern Minnesota is home to several experiments in particle physics and cosmology. I’ve written about one of the projects there, known as the Cryogenic Dark Matter Search (CDMS). Another experiment is Main Injector Neutrino Oscillation Search (MINOS), which detects muon neutrinos produced at Fermilab in Northern Illinois. MINOS is about 48 feet long, and contains 6000 tons of steel layered between scintillators. The entire detector had to be lowered down a narrow mine shaft piece by piece and then assembled on site.

The mine is located near Tower-Soudan, which happens to be close to where family live. So yesterday I was able to visit the physics lab, located more than 2000 feet underground. The mine has daily tours during the Summer, so if you find yourself in the Northwoods of Minnesota, it’s worth making a visit.

The post Nerds Seeking WIMPs appeared first on One Universe at a Time.

From measurements of distant supernovae, we now know our universe is not only expanding, but that it is expanding at an ever increasing rate. This cosmic acceleration is driven by what we call dark energy. While we can see the effects of dark energy, and we know it makes up about 68% of our universe, we don’t really know what dark energy actually is. That means while the experimentalists scurry to get more data, the theorists work frantically to explain what’s going on.

In a recent paper in the journal Astronomy and Astrophysics, an interesting theory was proposed to explain dark energy as a quantum effect. The idea is based on an experiment known as the Casimir effect. In quantum mechanics there are energy fluctuations at very small scales. Normally we don’t notice these fluctuations because they average out at larger scales. But you can observe their effect if you constrain the fluctuations. Experimentally this is done by placing two conducting surfaces very close together (on the order of microns). Between these surfaces the fluctuations are limited by the space between the surfaces, but elsewhere the fluctuations aren’t limited. As a result, the conducting surfaces are pulled toward each other, even though there is “nothing” between them.

What the Casimir effect demonstrates is that quantum fluctuations in a vacuum can produce a force. So what if dark energy was due to vacuum energy fluctuations? For that to be the case, there would have to be a constrained geometry. But such a constrained geometry is exactly what is proposed in (wait for it) string theory! String theory (or M-theory as it is now known) proposes that there are actually ten spatial dimensions instead of just the three that we know of. These higher dimensions differ from our usual ones in that they are compact, meaning they are folded around themselves. If you think of a straw as a two-dimensional sheet with one “regular” dimension (the length of the straw) and one “compact” dimension (the circumference of the straw), then you have the basic idea.

Now if these compact higher dimensions exist, then quantum fluctuations would be constrained in those dimensions, and that might produce a cosmic Casimir force that looks like dark energy. The catch is that our observations of gravity place a limit on the size of these compact dimensions. This is because gravity is an inverse-square force, meaning that the force of attraction between masses varies by one over their distance squared. If there are higher dimensions, then the inverse-square relation for gravity would break down at the size of those dimensions. Gravity experiments have shown no such effect, so that means that any higher dimensions (if they exist) can be no larger than 85 microns, which is about the width of a human hair.

This is where things get interesting, because if you calculate the effect of these quantum fluctuations in such compact dimensions you find that having 8 compact dimensions (as string theory proposes) doesn’t work. It only works if you allow for just one compact dimension. But if you constrain the universe to just one extra and compact dimension, and you also constrain the fluctuations to exist within the limits of the observable universe, then this model makes two predictions. The first is that dark energy would be constant throughout the universe. It would, in fact, look like a cosmological constant. The second is that this extra compact dimension would have a size of about 35 microns, which means that around 35 microns we should see gravity deviate from the inverse square relation.

Of course all of this could be wrong, but it is an interesting idea with predictions that should be testable in the near future. Sometimes it takes a bit of theory as well as experiments to solve a cosmic mystery.

The post The 5th Dimension appeared first on One Universe at a Time.

Yesterday while doing an AMA on Reddit I was asked an interesting question.  How do we know that dark energy, which is the attributed cause of cosmic expansion, isn’t just gravitationally repulsive matter?  After all, electric charges can attract and repel, so why not masses?  The repelling masses would push against each other and expand the universe.  How do we know that isn’t the cause of dark energy?  The short answer is that the observational evidence doesn’t support the idea.  But it is an interesting demonstration of how you can build and test ideas.

Let’s start with the idea that mass is somehow similar to electric charge.  In electromagnetism, there are positive and negative charges, and they follow the rule that charges of the same kind repel each other, while opposite kinds attract.  With gravity, masses of the same kind attract each other, so we could suppose that there is positive and negative mass, where masses of the same kind attract each other and opposites repel.

With charge, opposite kinds tend to come together to form objects that are electrically neutral.  The atoms in our body, for example, have positive and negative charges, but they tend to be distributed pretty evenly.  For our hypothetical positive and negative mass, you would tend to get clumps of one kind or another.  So we might speculate that some galaxies are formed of positive matter, while others are formed of negative matter.

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

That’s an interesting idea, but if that were the case, positive mass galaxies would attract other positive mass galaxies, but repel negative mass galaxies.  The same would be true for negative mass galaxies, so we would expect to see galaxies tend to differentiate into clumps of different mass types.  What we actually observe is that galaxies always attract each other.  The clustering of galaxies is consistent with them always attracting each other gravitationally, and there is no evidence of a differentiation.  So that idea doesn’t work.

So let’s try another idea.  Two types of mass, one being the regular kind that gravitationally attracts all other masses, and a second kind that always repels all other masses.  At first blush this might seem to be a good idea.  Regular matter would clump into galaxies and clusters, just as we observe, and this new repulsive matter would tend to spread out evenly and push against all the galaxies.  So you would get clumping and cosmic expansion just like we see.  Right?

Not quite.  The good news is this idea makes a clear prediction.  If cosmic expansion is due to some repulsive matter, then as the universe expands the density of that matter would decrease over time.  If the repulsive matter is more thinly spread, then its effect would be lessened.  So the clear prediction is that cosmic expansion is slowing over time.

This isn’t what we see, so this idea doesn’t work either.

What we actually see is that the universe is  expanding at an exponential rate.  This means that the rate at which the expansion occurs doesn’t decrease.  It is constant.  As the universe expands, the density of dark energy doesn’t decrease.  In general relativity this means that dark energy is a property of spacetime itself.  It cannot be some form of repulsive matter because the effect of cosmic expansion is persistent and unchanging.  Observations of distant galaxies confirm this.

So while we aren’t entirely sure what dark energy is, we do know unequivocally that it is not matter.

The post Energy Matters 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.