“Much later, when I discussed the problem with Einstein, he remarked that the introduction of the cosmological term was the biggest blunder he ever made in his life. But this “blunder,” rejected by Einstein, is still sometimes used by cosmologists even today, and the cosmological constant denoted by the Greek letter Λ rears its ugly head again and again and again.” -George Gamow, the father of the Big Bang model
The Big Bang — the prediction that the Universe started from a hot, dense, rapidly expanding state — tells us where our cold, star-and-galaxy-rich, slowly expanding Universe full of matter and energy comes from.
It also gives us a very exquisite prediction: because the Universe was once in a hot, dense state, there ought to be a tremendous amount of radiation left over from that. But because the Universe has expanded and cooled, that radiation should be extraordinarily cool and diffuse: just a few degrees above absolute zero!
But if we build a telescope (or satellite) capable of detecting those specific wavelengths of light, we should be able to see that light today! Well, with one caveat.
We see the light as it was emitted from when the Universe was just 380,000 years old — and the expansion-and-cooling allowed neutral atoms to form for the first time — but it has to pass through everything in the Universe in order to to reach us! So that means that not only are all the stars, gas and dust in our galaxy in the way, but so is all the matter and energy in our observable Universe!
So there were two challenges that needed to be met. One was to remove all the foregrounds — across all seven different wavelength bands — coming from our galaxy. And then, once that was accomplished, we could view just how all the matter in the Universe, as seen by Planck, was distributed.
That’s interesting in its own right, but today we’re interested in the light that came from the Big Bang. Once all of that is understood, we can take a look at the uniform background temperature of the Universe — confirmed to be 2.7255 K — and look at the tiny, microKelvin-scale fluctuations in it!
These fluctuations, and how they’re distributed, tell us an unprecedented amount about where the Universe came from as well as what’s in it. Without further ado, let’s take a look at Planck’s baby picture of the Universe!
Yesterday, I told you about what we knew about the Universe the day before Planck’s data release, but this picture is far more exquisite than any that came before. In the early 1990s, the COBE satellite gave us the first precision, all-sky map of the cosmic microwave background, down to a resolution of about 7 degrees. About a decade ago, WMAP managed to get that down to about half-a-degree resolution.
But Planck? Planck is so sensitive that the limits to what it can see aren’t set by instruments, but by the fundamental astrophysics of the Universe itself! In other words, it will be impossible to evertake better pictures of this stage of the Universe than Planck has already taken.
But what’s really amazing to me isn’t the picture itself, but what we learn about the Universe from it! Yesterday, I told you some things we hoped to learn, but as of right now, there are 30 Planck papers online right now! I’ve sifted through all of them for you, and here are the biggest, most important things we’ve learned! (According to me, anyway.)
The Universe has more matter and is expanding more slowly than we previously thought! If you had asked me yesterday I wouldn’t have predicted the Hubble constant to be this low. In fact, I did say this:
It probably couldn’t be as low as 60 or as high as 80, but no one would be shocked if it turned out to be 68 km/s/Mpc, or maybe as high as 74 km/s/Mpc. This could mean a Universe as old as maybe 14.2 billion years, or as young as 13.3 billion years, depending on how the dark matter and dark energy parameters adjusted.
Well, if you can remember all the way back to 2001, the Hubble space telescope, whose primary mission was to measure the Hubble constant, had said it was 72 ± 7 km/s/Mpc. Unsurprisingly, all the numbers that has come in afterwards were around that value: 72. But Planck has come in and said something different.
A Hubble parameter of just 67.3 ± 1.2 km/s/Mpc, which is amazing! This is significantly lower than what we had previously expected, although it was clearly still within the realm of possibility. Also, remember the way that error bars work: based on the Planck data, this means there’s a 68% chance that the Hubble parameter is between 66.1 and 68.5, and a 95% chance it’s between 64.9 and 69.7. So even though it’s smaller, it’s not necessarily significantly smaller. But still, that’s enough to be interesting and perhaps the biggest surprise!
How do we arrive at this conclusion? The “map” from Planck allows us to correlate the fluctuations on different scales. We can then put different parameters into our simulations — assuming General Relativity is correct — and see what parameters best match up with Planck’s observations.
The biggest surprise was that we found that the Hubble parameter was lower than we had previously thought. On its own (as in, if that were the only thing that were different), that would have made the Universe nearly a billion years older than we had previously thought!
But Planck gave us a couple of other, smaller surprises, too.
The amount of dark energy in the Universe is appreciably less than we had previously thought, while the amount of dark-and-normal matter is appreciably greater than we thought! Instead of a Universe made up of 73% dark energy, the best post-Planck estimate puts it at just 68-to-69%. Instead of a Universe with around 22% dark matter, Planck put that figure at more like 26-to-26.5%. And instead of 4.6% of the Universe (including us) being made of normal, baryonic matter (i.e., standard-model particles), Planck pushes that number up to 4.9%!
(And for those of you wondering, there’s still no spatial curvature observed.)
In other words, there’s slightly more normal matter, significantly more dark matter, and significantly less dark energy than we’d previously thought! So while the smaller expansion rate tells us the Universe is older than we’d previously thought, the increase in matter (and decrease in dark energy) makes the Universe younger than it would be otherwise!
A Universe that was 100% normal-and-dark matter would only be around 10 billion years old, but ours looks to be split about 31.5%-total-matter / 68.5% total-dark-energy. So when we put all of the best-fit data together, we get a Universe that’s 13.81 billion years old, or around 80 million years older than our previous best-estimate.
There are some things we’d definitely need to check with a Universe that’s tweaked in this way. First off, you might worry about whether this fit with what we already have measured about the large-scale clustering of galaxies and structure in the Universe. Well, Planck’s checked that against the new, best-fit parameters.
This still checks out within all the measured error-bars, which is a great sanity check of the results. You might also remember that there was some speculation that there might be an extra (or sterile) neutrino species out there; some of the WMAP results indicated that there could be, even though that was unexpected.
Well, it turns out that the Planck data puts the kibosh on that.
They even found out that this is something the slightly lower Hubble constant helps with; if we forced the Hubble constant to be 72, we would favor 4 neutrinos instead of 3, but the data doesn’t lie! Instead, what we find is that there are likely 3 neutrinos, the sum of their masses is very small (less than 0.18 eV), and there’s no evidence at all for a sterile neutrino species out there.
As far as inflation goes, Planck was looking for signatures of primordial gravitational waves, which could signal certain models of inflation, or rule others out. I wrote a little about what various models predict, but the best constraints would have come from the polarization data, which Planck is still analyzing. So that wasn’t part of the data release.
Still, what they were able to release put some pretty impressive constraints on various inflationary models, and confirmed that hybrid inflation is disfavored, and also put many models of chaotic inflation on notice.
I’ve included the traditional “new” inflation (or slow-roll inflation) models here, which predict tiny, tiny tensor modes that will probably never be observable. But even though this doesn’t give us the “smoking-gun” detection of tensor modes that would allow us to confirm a model of inflation, one of the best predictions of inflation is a scalar spectral index (referred to here as a primordial tilt) that’s almost but slightly less than one.
The power spectrum, illustrated below, clearly shows that Planck can (and has) discriminated between the inflationary prediction and, say, an untilted spectrum (which would be ns = 1).
So yes to inflation, no to gravitational waves from it.
Yes to three very light, standard-model neutrinos, no to any extras.
Yes to a slightly slower-expanding, older Universe, no to spatial curvature.
Yes to more dark matter and normal matter, yes also to a little less dark energy.
And as far as anything bizarre goes? The fluctuations are still very, very much in agreement with what inflation and all known physics predicts, but there’s still that very bizarre alignment of the CMB on the largest angular scales with the plane of our Solar System, known as the axis-of-evil.
Feel free to dive in to the papers yourself if you like, and stay tuned for the eventual polarization results, which will tell us more about inflation than anything else that’s ever come before!
It’s a great time to be alive, as we’re understanding the Universe better and better than ever. Thanks to the ESA, the Planck mission and the entire science team for bringing us these fabulous results, and a new, more accurate perspective on our Universe than we’ve ever gotten before!