When a black hole interacts with surrounding gas and dust, it can create jets of material that fly away from the black hole at nearly the speed of light. When those jets interact with the cosmic microwave background, something brilliant can occur. 

The cosmic microwave background (CMB) is an afterglow of the big bang. We see it as a faint glow of microwaves in all directions, and that’s because the entire universe is filled with the remnant light of the big bang. Photons from the CMB are streaming through the universe in all directions. When electrons in a black hole jet collide with these photons, they can give the photons an energy boost. Since the electrons collide at nearly the speed of light, the photons are boosted into x-rays.

Recently the Chandra x-ray observatory found an example of such a CMB enhanced jet. The x-rays from this jet began their journey more than 11 billion years ago. At this time the CMB was stronger, and so the x-rays emitted by this jet are about 150 times brighter than a jet forming in the present universe would be. It’s a great example of how the background light of the universe can be rekindled by black holes.

Interestingly, this active black hole isn’t bright at radio frequencies. Often the electrons of a black hole jet interact with magnetic fields to create strong radio emissions, and usually such black hole jets are detected by radio waves first. If more black holes can be x-ray loud but radio quiet, it could mean that there are many more distant x-ray jets that have simply been overlooked.

Paper: A. Simionescu, et al. Serendipitous discovery of an extended X-ray jet without a radio counterpart in a high-redshift quasar.  arXiv:1509.04822 [astro-ph.HE]

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Yesterday’s post on the sound of the big bang generated quite a bit of discussion about the cosmic microwave background (CMB), how it’s measured, and what it tells us. One of the challenges in interpreting the CMB data is that it’s very heavily processed. Some skeptics argue it’s too processed, so today I thought I’d talk about how we go from raw data to a detailed study of the early moments of the universe.

Early observations of the background (open circles) vs a theoretical blackbody. Credit: Brooks/Cole

When Penzias and Wilson discovered the CMB in 1965, they didn’t create an image map. What they did was to show that in all directions of the sky there is a uniform microwave background of a particular intensity. This intensity was in agreement with the prediction of a uniform thermal background due to the big bang. Over the next couple of decades, both ground based observations and observations made from high altitude balloons found that the cosmic background did indeed seem to follow the intensity curve of a thermal blackbody with a temperature of about 3 K. Since our atmosphere absorbs quite a bit of the shorter wavelengths of the blackbody curve, it was hard to pin down the temperature too precisely. To do that we needed to go beyond the atmosphere.

The COBE results at different sensitivities.

A big breakthrough came in 1989 with the Cosmic Background Explorer (COBE). This was a space-based observer that made a high-resolution survey of the sky. As expected, COBE observed a very uniform thermal background with a temperature of 2.728 K. The observations matched a blackbody curve more precisely than any experiment done thus far. If you plot the results as a temperature projection, what you get is a uniform temperature across the entire sky give or take a few thousandths of a Kelvin. We finally had definitive proof of the big bang. But COBE was designed not only to measure the overall temperature of the sky, but also to measure small variations in temperature at different points in the sky. From this we could determine not only the existence of the big bang but some of its details.

If you subtract out the average temperature from the CMB data, what you’re left with are variations on the order of milliKelvin. One of the first things we notice is that about half the sky is a bit warmer than the average, and about half is a bit cooler. You can see this in the projection maps of the relative temperatures across the sky. The yin-yang shape seen in the map is a dipole distribution, and it’s due to the motion of the Earth relative to the cosmic background. If the Earth is moving toward an object, the light we observe is shifted slightly toward the blue end of the spectrum (shorter wavelengths). For the CMB this means it looks a bit warmer than it actually is, since warmer objects emit shorter wavelengths. Likewise, if the Earth is moving away from an object, its light is shifted toward the red (longer wavelengths) and as a result the CMB looks a bit cooler in that direction. This shift of wavelengths due to relative motion is known as the Doppler effect, and is a basic tool of modern astronomy. From this dipole moment we find that galaxy is moving about 360 km/s relative to the cosmic background, which is actually faster than we expected.

CMB at different resolutions.

If you subtract the dipole moment from the data, then you can see fluctuations on the order of microKelvin. This is where the real meat of the CMB data is. Unfortunately at this level the interference from the Milky Way is a major source of the fluctuations. You can see it as a warm band across the center of the map. This is because we are within the Milky Way, and our view of the deep sky is somewhat obscured. To overcome this problem we have to distinguish between light coming from things like galaxies (called the foreground) and light actually coming from the cosmic background. To do this we observe the sky at several wavelength ranges. Foreground objects have a different spectral distribution than the cosmic background, so by observing the sky at several ranges we can make accurate maps of the foreground in order to subtract it from the data. The COBE satellite mapped the sky at three different ranges. WMAP made five maps in 2003, and Planck made nine sky maps in 2013. With more and more detailed foreground maps, we can more accurately subtract the “noise” from the real data.

Credit: Wayne Hu

With an accurate map of the fluctuations in the CMB we can then study the scales at which fluctuations occur. In mathematical terms we expand the ripples into a sum of multipole moments, of which the dipole is only the first. 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. From this we can create a power spectrum showing the strength of fluctuations at each scale. Within this power spectrum there are several peaks. The first three peaks of the spectrum tell us about the structure and history of the cosmos. The first peak tells us how flat the universe is (perfectly flat to the limit of observation). The second tells us the amount of regular matter in the universe (about 4.9% of the total mass-energy) and the third tells us the amount of dark matter (about 26.8% of the mass-energy). From the CMB we have moved beyond simply proving the big bang, and are now studying the details of cosmology. We have other sources of data beyond the CMB, and they generally agree with the results of the cosmic background.

At higher and higher multipoles (or smaller fluctuations) the foreground becomes increasingly difficult to remove. Gas and dust throughout the universe becomes a significant problem. This is part of the reason why it’s been so difficult to find evidence of cosmic inflation and why the results of BICEP2 were so controversial. It remains to be seen whether we can pull ever more signal out of the noise, and whether that will be enough to answer some of the basic questions of the origin of the universe. But even if we fail, we’ve come an astonishingly long way toward understanding our universe.

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The cosmic microwave background (CMB) is the thermal remnant of the big bang. It was once a warm glow of about 3,000 K, but as the universe expanded and cooled the CMB gradually chilled to about 2.7 K. As a result, its once orange glow has shifted down to the microwave range, hence its name. Usually the CMB is presented as an image, where various colors represent the small fluctuations in the background, but when it was first observed it was detected as sound.

Because the CMB is in the microwave range, you can convert its electromagnetic waves to sound just as we do with radio. The result is a kind of faint static you can hear above. The sound was captured by Robert Wilson, who with Arno Penzias made the first measurement of the CMB as a blackbody background. It might not sound like much, but the very fact that it’s there was the first definitive proof of the big bang. Throughout the entire universe that faint sound can be heard. It’s everywhere and in all directions.

People often imagine the big bang as beginning with an explosion expanding out of darkness, but that faint hiss shows us that isn’t true. The big bang happened everywhere, and everything we see around us is a product of it.

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One of the common accusations made about cosmology is that it is just a kludge model to explain away all the strange things we see in the universe. Galactic redshift? Invent the big bang. Galaxies don’t move the right way? Invent dark matter and dark energy. Tweak any model enough and you can make it fit data. Part of the reason for this is the way scientific discovery is represented. The lone genius has a revolutionary idea that clears away all the stuffy old models. But that’s not how science gets done. Scientific models are often proposed to explain strange data, but the real test is whether the predictions they make hold up under scrutiny. Take, for example, the story of the cosmic microwave background.

The cosmic microwave background (CMB) is often cited as definitive proof of the big bang. Discovered in the 1960s by Penzias and Wilson, it has since been used to understand the evolution of the cosmos, aspects of dark matter and dark energy, and might even tell us about early cosmic inflation. But the big bang wasn’t invented to explain the CMB, rather the CMB was a prediction made twenty years before its discovery.

The first proposal for a “big bang” model is typically attributed to a 1931 paper by Georges Lemaître. Based on the properties of general relativity, Lemaître argued that the universe must have began as a “primeval atom.” The idea didn’t attract much attention because the “static universe” was the dominant model at the time. When Edwin Hubble demonstrated a correlation between a galaxy’s distance and its redshift, it became clear that the universe was expanding. At that point Lemaître’s idea gained attention. Still, there was opposition to the idea, mainly because it was seen as extending the evidence too far. After all, if we watch bread dough rise it is clear that the dough is expanding, but to conclude from this that bread begins as an ultra-dense “primeval flour” is ridiculous.

Then in 1948 Ralph Alpher, and Robert Herman published a paper predicting a consequence of the big bang model. The paper was actually focused on the relative abundance of elements in the universe, but it noted that if the universe began hot and dense as the big bang model claimed, then there must be a thermal remnant. That is, the universe must be bathed in thermal microwaves from the big bang, and the spectrum of that background must match that of a blackbody. In the paper they estimate the temperature of that background to be about 5 Kelvin.

Twenty years later, it was found that the universe is indeed bathed in a microwave background, with an almost perfect blackbody spectrum at a temperature of about 3 Kelvin.

Paper: Alpher, R. A.; Herman, R. C. On the Relative Abundance of the Elements. Physical Review 74 (12): 1737–1742 (1948).

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In the standard ΛCDM model of cosmology, the early universe was in a hot dense state known as the big bang. As the universe expanded, its density and temperature dropped to the point where electrons and nuclei could combine to form neutral hydrogen and helium (with small traces of other elements). At that point the light of the universe was finally free to travel great distances through space without colliding with ionized particles. We see that first light as the cosmic microwave background.

After that period (known as recombination) there was no way for new light to be created. The primeval fireball had become too cool to produce new light, and there were no stars to shine. As a result, the universe entered a period known as the dark ages, that spans the time between recombination and the formation of the first stars. Just how long that dark age lasted has been difficult to pin down.

Stars re-ionize material, which interacts with the cosmic background. Credit: ESA

Based on observations such as the Hubble Deep Field, it’s been estimated that the dark ages ended about 400 million years after the big bang.  But new observations from the Planck satellite gives us a much more accurate age of 550 million years after the big bang. This is possible by looking at the polarization of light from the cosmic microwave background. In the last moments before recombination, when the electrons finally started to bond with nuclei, the photons created in the big bang would have one last scatter off an electron before making its long journey across the universe. Because of this the light is polarized, which is one of the things Planck has studied in detail. Throughout the dark ages, that polarization was “fixed” since there was no free electrons for the light to scatter off. But once stars began to form, the new light and heat could re-ionized surrounding material. As a result, some of the light from the cosmic background was scattered again, polarizing it in a different way. This new scattering leaves its fingerprints on the CMB, which tells us when it occurs.

What’s interesting about this new result is that by the time the dark ages ended, stars and galaxies were already forming. This means that by the time stars began to shine the structure of galaxies was already set into motion. The dark ages lasted longer than we though, but it also ended more abruptly than we suspected.

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In earlier posts about dark matter, I’ve written about how neutrinos would seem to be a good candidate, but there simply aren’t enough of them to account for all of dark matter. As far as we can tell, there are three types (flavors) of neutrinos, and we know the upper limit of their mass from the distribution of galaxies in the universe. So the three known neutrino flavors can’t be the solution to dark matter.

But there has been speculation that a fourth type of neutrino could exist, known as a sterile neutrino. Sterile neutrinos wouldn’t interact through the weak nuclear force the way regular neutrinos do, but instead only interact with things gravitationally (hence sterile). If these neutrinos had a much greater mass than regular neutrinos, then it could be an answer to the dark matter problem. But the catch is that their gravitational interactions would have interacted gravitationally with matter in the early universe, and this would affect the fluctuation patterns in the cosmic microwave background. So in principle we should see their effect in CMB fluctuations.

Previously the WMAP observations of the CMB were inconclusive. The data agreed with the 3-neutrino model, but the effect of a sterile neutrino couldn’t be excluded. But the latest Planck results now confirm that there is no fourth neutrino. The  standard model of particle physics remains successful. This is good news for the standard model, but it eliminates one more dark matter candidate from the list.

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More results from the Planck collaboration are coming in, this time from conference in Italy. There’s been a lot of excitement building up to this new release, particularly given some of the unresolved issues in the standard model of cosmology.

One of these issues is the tension between the earlier WMAP data and the Planck data regarding the average temperature of the universe. Early indications were that they differed by about 1.5%, which isn’t huge, but was concerning given the precision of both data sets. The new analysis finds they agree within 0.3%, which is within the uncertainty range of Planck. So, nothing new or interesting there.

Another result puts further constraints on parameters for dark matter. The new results are precise enough to eliminate some dark matter models. Back in April of this year there was a big announcement that the AMS detector on the space station had detected a possible dark matter signal. The bold claim wasn’t substantiated by the data, and as a result it met with quite a bit of criticism. The new Planck data confirms that the AMS claim was wrong. Again, this is what most of us expected.

The fluctuation data matches theory extremely well.
Credit: ESA – Planck collaboration.

The one interesting thing about the new results regards what wasn’t mentioned. What about BICEP2 and the issue of whether its signal was due to interstellar dust or evidence of early cosmic inflation? So far there’s been no announcement. In fact, in the released graphs on the fluctuations of the cosmic background, and the smallest scale fluctuations aren’t shown. That’s where dust becomes an issue. Just how much of an issue remains to be seen.

But at larger scales the Planck data matches theory astoundingly well. Of course WMAP matched it exceptionally well too. Planck just added a bit more precision to what we already knew. So for now it seems rather ho-hum. Nothing really new to see, no sensational headline to be found.

But I just can’t see it that way. Because this latest data confirms that we understand the origin and evolution of the universe really well. A bunch of humans standing on a rock have come to understand the cosmos. Our theory works, and it keeps working. This is an absolutely astounding achievement.

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This month Astronomy & Astrophysics released 31 articles on data gathered by the Planck satellite. This includes nine all-sky surveys at a range of wavelengths from radio to infrared. It represents the most detailed map of the cosmic sky to date, and already there are some interesting results.

What makes the various sky-surveys useful is that each different wavelength gives a different view of the sky. For example, at the 1 cm range the radio emissions of our Milky Way galaxy tend to dominate. At the 300 micron level is the far infrared, which is where the signal of the infamous cosmic dust is strongest. In the middle range (around the 2 millimeter wavelength) is where the cosmic microwave background dominates.

From all these we can better distinguish the foreground effects (all the stars, galaxies, dust, etc.) from the distant cosmic background itself. This not only gives us a clear image of the cosmic background, it also gives us lots of data on all the foreground stuff. This isn’t just noise to be discarded. It actually helps pin down various cosmological parameters.

One of these parameters is the Hubble constant, which determines the rate of cosmic expansion. When we look at the redshift of galaxies, much of it is due to the cosmic expansion, but some is due to the motion of the galaxies through space (which is different). From redshift alone we can’t tell the difference, but the motion of galaxy clusters distorts the cosmic microwave background through what is known as the SZ effect. Basically, low energy photons from the cosmic background collide with fast moving electrons in a galaxy cluster. The photons are then scattered with a great deal of energy, thus changing their wavelength. We can see this as a shift in the wavelength distribution of the cosmic background in the region of the cluster. By measuring this effect we can distinguish between cosmic redshift and Doppler redshift.

Of course the big goal is the detailed mapping of the cosmic background itself, and this new work looks at that in detail. A few of the papers look at the possibility that the background isn’t quite the same in all directions, something that has been hinted at in earlier data, and seems to still be hinted at in this new data. Then of course there is the issue of early cosmic inflation, which entered the news when BICEP2 announced evidence of its existence, then had to tone down some of its claims. These new results put the strongest constraints on cosmic inflation thus far.

There’s a lot here. Far more than can be covered in a single post. As I work through the papers I’ll have much more to say. But that will have to wait for another day.

Papers: A&A special feature: Planck 2013 results. Astronomy & Astrophysics, volume 571, November 2014.

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Polarization in the cosmic microwave background (CMB) is in the news again. This time with new results from a project known as POLARBEAR. The results were published this week in the Astrophysical Journal, and it swings the observational needle back towards the existence of cosmic inflation.

The evidence for cosmic inflation has been very publicly playing out in the media. It began with BICEP2 announcing that they had discovered evidence of primordial gravitational waves, which is a signature of early cosmic inflation. After their public announcement there was a flurry of criticism, particularly on whether BICEP2 had eliminated the effects of dust from their data. The BICEP2 paper did make it through peer review, but then the Planck team showed pretty clearly that dust was still a problem. It began to look like perhaps the entire “signal” of BICEP2 was due to dust.

Now POLARBEAR has tossed their hat into the ring. Their paper is more cautious, and they waited until surviving peer review before announcing it. As with BICEP2, it all comes down to dust, and whether its effects can be eliminated from the data.

Different modes of CMB polarization.
Credit: Sky and Telescope

Like the other projects, POLARBEAR has been observing polarized light within the cosmic microwave background, specifically a type of polarization known as B-mode polarization. The CMB is polarized due to the small fluctuations in temperature and density that existed at the time. This primordial polarization is known as E-mode polarization. If there was cosmic inflation in the early universe, then the primordial gravitational waves of inflation would distort some of the E-mode into B-mode polarization.

It would seem then that any detection of B-mode polarization is evidence of primordial gravitational waves, and thus of inflation. But the problem is that lots of other things can also create B-mode polarization. One of the big alternative sources is dust. The cosmic microwave background is the most distant light we can observe, so all the galaxies, dust, interstellar plasma and our own galaxy is between us and the CMB.  if you don’t account for them properly, then you could get a “false positive.”

The way POLARBEAR dealt with this challenge is two-fold. First, they took data from regions of the sky where dust contamination is at a minimum. Second, they gathered data at a higher resolution. This latter part is important, because at higher resolutions B-mode signals from gravitational lensing should be more dominant than those due to dust. They then did something rather interesting. Taking their data they tried to see if they could subtract out all the contaminating sources to get a null result.  In other words, started with the assumption that everything was a contaminant, and then tested whether that model worked.  What they found was that it didn’t. Specifically, they excluded the null result model with 97% certainty.

So their data showed that it’s not nothing. Within the B-mode data there is a portion of it that is due to gravitational lensing. It isn’t all due to dust and contaminants, as some have feared. So does that mean they found evidence of early inflation? Not quite. They’ve shown that they can see a gravitational signal, but they haven’t proven that the signal is due to primordial gravitational waves. The authors are being cautious here, which is probably a good thing.

So there’s hope that we will be able to observe the fingerprint of inflation (assuming it occurred). It just keeps getting more interesting.

Paper: P. A. R. Ade et al. A Measurement of the Cosmic Microwave Background B-Mode Polarization Power Spectrum at Sub-Degree Scales with POLARBEAR. ApJ 794 171 (2014)

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One of the big questions in cosmology regards the shape of the universe. “Shape” in this case is not the distribution of galaxies, but rather the shape of space and time itself. In general relativity, space and time can be warped by masses (producing the effect of gravity), and it can be warped by dark energy (producing cosmic expansion). Knowing the shape of the cosmos lets us determine if it is finite in size or infinite, and whether it will expand forever or collapse back upon itself.

It’s generally thought that the universe is “flat”, meaning that if you took an average of a the gravitational and dark energy curvatures, it would balance out to zero. In a perfectly flat universe, two laser beams aimed parallel to each other would remain parallel forever. If the universe were not flat, then the laser beams would eventually diverge or converge. Compare this to the surface of the Earth, which locally appears flat, but on large scales is a curved surface. If two people standing side by side started walking due north, they would eventually converge at the north pole.

We see evidence of this flatness in the cosmic microwave background (CMB). Overall, the temperature of the CMB is very uniform. There are small variations in its temperature, but those small variations average out. This is exactly what we would expect with if the universe is (on average) flat. If the universe had some overall curvature, it would appear in the CMB as an asymmetry in the overall temperature. For example, one whole section of the background would be colder or warmer than expected by random chance.

But recently we’ve found that there is a bit of an anomaly in the CMB. This result has been a bit controversial, because while it is a real and measured effect, its statistical significance is fairly small. The anomalies exist at a level of 3-sigma, which means there is only a 1% chance that they are due to random variations. In astrophysics we like to have new results at a 5-sigma level, or 1 in a million odds. So there are some who see this anomaly as a hint of some kind of new and exotic physics, while others (myself included, truth be told) who think we shouldn’t read too much into it until we have stronger data.

While many proposals to explain this anomaly invoke exotic ideas such as multiverses, a recent paper in Physical Review Letters shows that the anomaly could be explained entirely by cosmic curvature. The team has demonstrated that the observed anomalies (if real) can be accounted for by a universe with a saddle shaped universe, seen as the left figure in the image above.

If this is true, then it we will have to re-examine our understanding of the early universe. Our current model of the early inflationary period predicts that the universe should be flat, and so far that has held up. If the universe actually is curved, then the inflationary period must have been more complex than we have thought.

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The cosmic microwave background (CMB) is the thermal afterglow of the primordial fireball we call the big bang. One of the striking features of the CMB is how remarkably uniform it is. Still, there are some small variations in temperature at various points in the sky. This is actually expected, and in fact the scale at which these fluctuations occur tells us a great deal about the structure of the universe. But there is also a fluctuation that isn’t expected, and its cause is a bit of a mystery. It is known as the CMB cold spot, and there has been much speculation as to its cause.

The CMB cold spot is not particularly colder than other cold regions of the CMB, but it is unusual because it is a particularly cold region surrounded by a rather warm region. Simulations of random fluctuations in a CMB estimate that the odds of such a cold spot happening in the universe is about 1 in 100. So it’s possible that it is just a random fluctuation. But the 1% odds is small enough that some astronomers have looked for a possible cause, and these ideas have ranged from the mundane to the wild.

One idea is that the cold spot is due to a large void in that particular direction. We know that galaxies cluster into clumps of galaxies separated by regions with very few galaxies. Since the cosmic microwave background lies beyond these galaxies, the light from the CMB must pass through regions of clusters and voids to reach us. The gravitational effects of clusters can make the CMB in that direction appear warmer, while the lack of gravitational interactions in voids can make it appear cooler.  This is a well-known phenomenon known as the Sachs-Wolfe effect. If there were a particularly large void (sometimes referred to as the Eridanus supervoid) it could explain the cold spot in that direction. There has been some evidence for such a void, but very recent (and yet to be peer reveiwed) work suggests that any such void isn’t large enough to produce the cold spot. So that doesn’t seem to be the solution.

A more speculative idea is that the cold spot is due to gravitational influences from a parallel universe. If there were such such a thing in a cosmic “multiverse” then the CMB would be a place to look. The claim has gotten a lot of press, but there’s little evidence to support it.  In particular, according to the model there should be a corresponding cold spot on the other side of the universe, but there doesn’t seem to be such a thing.

The most common explanation is that the CMB cold spot is just an odd random fluctuation. It may be unusual, but not so unusual that it actually needs an underlying cause. In fact, even the claim of it being statistically unusual can be debated. While some statistical methods show it to be unexpected, others don’t recognize it as unusual at all.  And we know that sometimes if you look for an unusual feature strongly enough you will eventually find one.

So while it is worth studying, it isn’t worth inventing parallel universes to explain.

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One of the big points of evidence in support of the big bang is the cosmic microwave background (CMB). It is often described as the afterglow of the primordial fireball, but it is much more than that. As we make better observations of the CMB we not only gather evidence of the origin of the universe, we also get an indication of the specific nature of our universe. One of the ways we see this is through what’s known as the three peaks.

When we look at the cosmic microwave background, one of its distinctive features is that it is a thermal blackbody. In all different directions, it has a temperature of 2.7 K. This is exactly what you’d expect if the universe was once in a hot, dense state.  While the CMB is extraordinarily uniform in temperature, it isn’t perfectly uniform. There are very small fluctuations in temperature. These fluctuations were mapped in detail by the COBE satellite in 1992. Later, more detailed maps of these fluctuations were made by the WMAP and Planck satellites.

Credit: Wayne Hu

These variations in temperature are like ripples on the cosmic pond. By observing these ripples we can determine what was happening in the universe when it was very young, which in turn tells us things about the universe today. One of the ways we do this is by looking at the scales at which temperature fluctuations occur.  You can see this in the figure, where the amount of temperature fluctuations (in microkelvin) is plotted against what is known as the multipole moment (l), which is a measure of the fluctuation scale. Basically, you 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 higher multipole scale. When this power spectrum of CMB fluctuations is plotted, you notice a big first peak, followed by two smaller peaks before it dies off in a series of small fluctuations. It is these three peaks that paint a clear picture of our universe.

Credit: moot Cosmology Group

The first peak is an indication of how flat or curved the universe is as a whole. Since the cosmic microwave background comes from the farthest edge of the visible universe, it’s light can be distorted by the cosmic curvature. If the universe is flat (like a flat sheet of paper) then the fluctuations we observe would appear undistorted. If it is positively curved (like the surface of the Earth) then the fluctuations would appear magnified by the cosmic curvature. If the universe is negatively curved (like the surface of a saddle) then the fluctuations would appear smaller.

On the power spectrum curve, this means the first peak would be more to the left if the universe is positively curved and more to the right if negatively curved. What we find is that the first peak is at about l = 200, which the value for a flat universe. Within the limits of our CMB observations, the universe is exactly flat. This means that the universe as a whole is at least 150 times larger than the observable universe. It may be infinite, but at the very least it is really, really big.

The second peak tells us about the amount of matter in the universe. Given the initial fluctuations in the universe, all matter would tend to gravitationally clump toward the higher temperature (higher density) fluctuations, which would tend to reinforce them on smaller scales. But regular matter (the kind that interacts with light) would also tend to heat up as it clumps, and the resulting pressure would tend to push against the clumping matter. So the more regular matter you have, the more pushback you would get. This means that the more regular matter there is in the universe, the more the second peak will be damped. So the smaller the second peak, the more matter in the universe. Given the observed level, we find that regular matter makes up about 4.9% of the mass/energy of the universe.

The third peak is an indicator of dark matter in the universe. Dark matter gravitationally clumps like regular matter, but since it doesn’t interact strongly with light, it isn’t affected by the pressure of light against it. So as the regular matter clumping experiences a push-back, the dark matter clumping does not. So the height of the third peak gives us a measure of how much dark matter there is compared to the total amount of light in the early universe (what’s known as the matter/radiation ratio). Since we know that light and regular matter are related, the matter/radiation ratio tells us the amount of dark matter in the universe. From this we find that dark matter makes up about 26.8% of the mass/energy of the universe.

The universe according to the CMB. Credit: ESA/Planck

Since the universe is flat despite having matter and dark matter, the remaining mass energy must be in the form of dark energy. Without it, the universe would appear positively curved (closed), which we don’t see.  So from observations of the CMB we find the universe is flat, about 4.9% regular matter, 26.8% dark matter, and 68.3% dark energy.  This is found simply from the cosmic microwave background. We have other observations, such as the redshift/distance relation of galaxies, the way in which galaxies clump in the present universe, and a variety of other tests. They all give similar results. In this way we have a confluence of evidence that clearly shows the size, age, and structure of the universe.

But even if all we had was the cosmic microwave background, we would still know quite a bit. And as we gather even more sensitive observations, it will surely tell us even more.

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