One of the basic assumptions of cosmology is that the Universe is basically the same everywhere. That is, our location in the cosmos isn’t special, and if we happened to be located in another corner of the galaxy we’d see basically the same thing. It’s sometimes known as the Copernican principle, since it was Copernicus who famously proposed the Earth was not the center of the Universe. But given that we haven’t traveled very far into space, how can we really know that assumption is valid? 

The usual condition for the Copernican principle is that the Universe is homogeneous (meaning that on large scales matter should be evenly distributed) and isotropic (meaning that there is no special direction or orientation in the cosmos). The first can be seen by observing the distribution of galaxies in the visible universe. On small scales galaxies are clumped into cluster and superclusters, but at increasingly large scales things even out. We also know that the laws of physics seem to be the same throughout the cosmos, which supports the idea of homogeneity.

But what about the directionality of the Universe? To be isotropic, all directions have to be equivalent. This means, for example, that the Universe as a whole can’t be rotating. If there was a cosmic rotation, then the axis of rotation would be a preferred direction. It also means that the Universe can’t be expanding more quickly in one direction, since that would give the Universe a specific orientation. These aren’t simply hypothetical constraints. As Kurt Gödel demonstrated in 1949, general relativity does allow for a rotating universe, and several models of cosmic expansion have proposed that it might vary for different regions of space. If it does turn out the Universe isn’t isotropic, then our current model of the Universe would be overturned.

One of the best ways to test isotropy is to look at the cosmic microwave background (CMB). Since the CMB is the most distant source of light, any rotation or preferred expansion would be seen as deviations from isotropy within the CMB. Previous studies such as that of the Planck spacecraft have found no evidence of any deviations, but a new study takes things one step further. It looked at everything from the distribution of hot and cold regions within the cosmic background to the polarization of CMB light (what’s known as vorticity). The study found the CMB is isotropic to the limits of the data. Specifically they found the odds of a preferred direction in cosmic expansion to be  121,000 to 1 against. This is good news for supporters of the standard model, as still theoretical models such as early cosmic inflation.

There might still be some small anisotropy within the Universe, but as far as we can tell Copernican principle holds. For now, cosmology continues to lack a sense of direction.

Paper: Daniela Saadeh, at al. How Isotropic is the Universe? Physical Review Letters, 2016; 117 (13) DOI: 10.1103/PhysRevLett.117.131302

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When we look out into the universe, we can see a cosmic background of microwaves in all directions. It’s the thermal remnant of the big bang, but it also serves as a wall beyond which we cannot see. Until now.

The early universe was extremely hot. So hot that any light emitted by particle interactions couldn’t travel far, much like light bouncing around the interior of our Sun. But by the time the universe was about 380,000 years old, its expansion cooled the universe to about 3,000 K, which is about the same as the surface temperature of a cool star. At that point the hydrogen and helium in the universe finally cooled enough to became transparent to light. The light of the universe could finally travel freely for billions of years. As the universe continued to expand, it cooled to the cosmic microwave background (CMB) we see today.

Since the background is from a time when the universe was 380,000 years old, we have no way of directly observing the earliest period of the universe. This doesn’t mean we can’t see anything. The CMB we observe isn’t perfectly uniform, but rather has small fluctuations in temperature. The scale at which these fluctuations occur is determined by various factors, including the amount of matter, dark matter and dark energy in the universe.  All the observations we’ve made of the CMB is consistent with the standard cosmological model, known as the LCDM model.

While light can’t be seen beyond the CMB wall, neutrinos could in principle. Since neutrinos only weakly interact with other matter, it could travel freely through the universe from the moment it formed. If we could observe these cosmic neutrinos we’d be able to see the universe when it was only a second old. Unfortunately, these primordial neutrinos would be far too cold and have too little energy to be seen anytime soon. From the big bang model they would be about 2 K, which is much less energy than current neutrino detectors can observe.

Observed fluctuations compared to neutrino number models. Credit: Brent Follin, et al.

But neutrinos also have an indirect effect on the fluctuations of the cosmic microwave background. Although they don’t interact much with other matter, they do interact slightly, and this can shift the fluctuations very slightly. In a recent paper, a team looked at CMB data from the Planck satellite, and they found just such a shift. The team compared this shift with different cosmological models where the number of neutrinos varied from 1 to 5. While we’ve long thought that there are just 3 different types of neutrinos, some speculative models predict more. What they found, pretty conclusively, is that the fluctuations agree with the 3-neutrino model.

So now we know cosmologically that there are just 3 types of neutrinos. The standard model of particle physics and the standard model of cosmology agree. Given how radically different the regimes of these models are, that’s pretty amazing indeed.

Paper: Brent Follin, et al. First Detection of the Acoustic Oscillation Phase Shift Expected from the Cosmic Neutrino Background. Phys. Rev. Lett. 115, 091301 (2015)

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Recently rumors have been flying that the BICEP2 results regarding the cosmic inflationary period may be invalid. It makes for great headline press, but the reality is not quite so sensational. There may be some issues with the BICEP2 results, but that isn’t what the press is excited about. What they are really excited about is how science groups are airing their dirty laundry, publicly. So what’s really going on?

For those who might not remember, BICEP2 is a project working to detect polarized light within the cosmic microwave background (CMB). Specifically they were looking for a type of polarization known as B-mode polarization. Detection of B-mode polarization is important because one mechanism for it is cosmic inflation in the early universe, which is exactly what BICEP2 claimed to have evidence of.

Part of the reason BICEP2 got so much press is because B-mode polarization is particularly difficult to detect. It is a small signal, and you have to filter through a great deal of observational data to be sure that your result is valid.  But you also have to worry about other sources that look like B-mode polarization, and if you don’t account for them properly, then you could get a “false positive.” That’s where this latest drama arises.

In general this challenge is sometimes called the foreground problem.  Basically, the cosmic microwave background is the most distant light we can observe. All the galaxies, dust, interstellar plasma and our own galaxy is between us and the CMB.  So to make sure that the data you gather is really from the CMB, you have to account for all the stuff in the way (the foreground).  We have ways of doing this, but it is difficult. The big challenge is to account for everything.

You might remember a while back I wrote about one foreground effect that BICEP2 didn’t take into account. It involves an effect known as radio loops, where dust particles trapped in interstellar magnetic fields can emit polarized light similar to B-mode polarization. How much of an effect this might have is unclear. Another project being done with the Planck satellite is also looking at this foreground effect, and has released some initial results, but hasn’t yet released the actual data yet.

Now it has come to light that BICEP2 did, in fact, take some of this foreground polarization into account, using results from Planck. But since the raw data hadn’t been released, the team used data taken from a PDF slide of Planck results and basically reverse-engineered the Planck data.  This isn’t ideal, but it works moderately well. Now there is some debate as to whether that slide presented the real foreground polarization or some averaged polarization. If it is the latter, then the BICEP2 results may have underestimated the foreground effect. Does this mean the BICEP2 results are completely invalid? Given what I’ve seen so far, I don’t think it does. Keep in mind that the Planck foreground is one of several foreground effects that BICEP2 did account for. It could be a large error, but it could also be a rather minor one.

Because of all this drama, there are already posts out there claiming that this is evidence of scientists behaving badly, or declaring this kind of thing shows that scientists don’t really know anything. But it is important to keep in mind that the BICEP2 paper is still undergoing peer review.  Critical analysis of the paper is exactly what should happen, and is happening.  This type of dirty laundry used to be confined to the ivory towers, but with social media it now happens in the open.  This is how science is done. BICEP2 has made a bold claim, and now everyone gets to whack at them like a piñata.

Now lets see if their result holds up, or falls apart.

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