Ask Ethan: What Could Solve The Cosmic Controversy Over The Expanding Universe?
“As you pointed out in several of your columns, the cosmic [distance] ladder and the study of CMBR gives incompatible values for the Hubble constant. What are the best explanations cosmologists have come with to reconcile them?”
If you had two independent ways to measure a property of the Universe, you’d really hope they agreed with one another. Well, the situation we have with the expanding Universe is extremely puzzling: we have about 10 ways to do it, and the answers all fall into two independent and mutually incompatible categories. Either you make the measurement of an early, relic signal that’s observable today, and you get a value of 67 km/s/Mpc, with an uncertainty of about 1%, or you measure a distant object whose emitted light comes directly to our eyes through the expanding Universe, and you get a value of 73 km/s/Mpc, with an uncertainty of about 2%. It’s looking increasingly unlikely that any one group is wrong, in which case, we absolutely require some new, exotic physics to explain it.
While many ideas abound, there are five of them that are eminently testable in the next decade or so. Here’s how we could solve the expanding Universe controversy in the best way possible: with more and better science!
This Is How We Know The Cosmic Microwave Background Comes From The Big Bang
“The outer layers are extremely tenuous and rarified, and the radiation we receive here on Earth doesn’t all originate from the very edge of that plasma. Instead, much of what we see originates from about the first 500 kilometers, where the interior layers are significantly hotter than the outermost ones. The light coming from our Sun — or any star, for that matter — is not a blackbody, but the sum of many blackbodies that vary in temperature by many hundreds of degrees.
It’s only when you add all these blackbodies together that you can reproduce the light we see coming from our parent star. The cosmic microwave background, when we look at its spectrum in detail, is a far more perfect blackbody than any star could ever hope to be.”
If you get your science from the internet, you might hear about all sorts of alternatives to the Big Bang. Grandiose claims are often made, decrying the Big Bang as a religion that can never be falsified, while simultaneously touting ideas that most scientists discarded decades or even centuries ago.
But there is no ideology at play; science is a game that we play with predictive power and evidence. The Big Bang makes explicit predictions, and so do alternative ideas that rely on atomic emissions, reflected starlight, photonic energy loss, or heated-up dust.
We can look at every idea we can conceive of, but in the end, only one matches what we observe. Here’s how the Cosmic Microwave Background points to the Big Bang, and away from every other alternative.
The Simplest Solution To The Expanding Universe’s Biggest Controversy
“This is how dark energy was first discovered, and our best methods of the cosmic distance ladder give us an expansion rate of 73.2 km/s/Mpc, with an uncertainty of less than 3%.
If there’s one error at any stage of this process, it propagates to all higher rungs. We can be pretty confident that we’ve measured the Earth-Sun distance correctly, but parallax measurements are currently being revised by the Gaia mission, with substantial uncertainties. Cepheids may have additional variables in them, skewing the results. And type Ia supernovae have recently been shown to vary by quite a bit — perhaps 5% — from what was previously thought. The possibility that there is an error is the most terrifying possibility to many scientists who work on the cosmic distance ladder.”
We live in an expanding Universe that’s 13.8 billion years old, full of two trillion galaxies, containing dark energy, dark matter, normal matter and radiation. For decades, we’ve been refining and better-understanding this cosmic picture, with one of the goals of modern astrophysics to measure the rate of expansion. Right around the year 2000, results from the Hubble key project, the scientific reason the Hubble space telescope was built, indicated that the expansion rate was 72 km/s/Mpc, with an uncertainty of around 10%. Now, we have multiple independent ways to measure that rate to even greater precision, but the problem is that two different groups no longer agree. One claims a rate of 73.2 km/s/Mpc, and the other claims a rate of 67.4 km/s/Mpc. The claimed uncertainties are small, and do not overlap.
Is this a crisis for cosmology? Or is one group simply mistaken due to an unidentified error? Is this a loose OPERA cable all over again? Here’s the big question keeping scientists up at night.
Why Cosmology’s Expanding Universe Controversy Is An Even Bigger Problem Than You Realize
“The question of how quickly the Universe is expanding is one that has troubled astronomers and astrophysicists since we first realized that cosmic expansion was a necessity. While it’s incredibly impressive that two completely independent methods yield answers that are close to within less than 10%, the fact that they don’t agree with each other is troubling.
If the distance ladder group is in error, and the expansion rate is truly on the low end and near 67 km/s/Mpc, the Universe could fall into line. But if the cosmic microwave background group is mistaken, and the expansion rate is closer to 73 km/s/Mpc, we just may have a crisis in modern cosmology.
The Universe cannot have the dark matter density and initial fluctuations that such a value would imply. Until this puzzle is resolved, we must be open to the possibility that a cosmic revolution may be on the horizon.”
Ever since we first learned that the Universe was expanding, scientists have worked hard to measure just how fast that expansion rate is. From that, combined with what makes up the Universe, we can learn how old the Universe is and what it was like in the past, as well as what it’s fate will be in the future. Yet the two groups that make independent measurements of that rate, from the cosmic microwave background and the cosmic distance ladder, have gotten inconsistent results. If the distance ladder team has made a mistake, everything will be fine with cosmology. But if that team is right and the microwave background team is wrong, there should be a crisis coming.
Why is that? Come find out why the biggest controversy in modern cosmology might be an even bigger problem than almost everyone realizes!
Ask Ethan: Will Future Civilizations Miss The Big Bang?
“If intelligent life re-emerges in our solar system in a few billion years, only a few points of light will still be visible in the sky. What kind of theory of the universe will those beings concoct? It is almost certain to be wrong. Why do we think that what we can view now can lead us to a “correct” theory when a few billion years before us, things might have looked completely different?”
Incredibly, the Universe we know and love today won’t be the way it is forever. If we were born in the far future, perhaps a hundred billion years from now, we wouldn’t have another galaxy to look at for a billion light years: hundreds of times more distant than the closest galaxies today. Our local group will merge into a single, giant elliptical galaxy, and there will be no sign at all of young stars, of star-forming regions, of other galaxies, or even of the Big Bang’s leftover glow. If we were born in the far future, we might miss the Big Bang as the correct origin of our Universe. It makes one wonder, when you think about it in those terms, if we’re missing something essential about our Universe today? In the 13.8 billion years that have passed, could we already have lost some essential information about the history of our Universe?
And in the far future, might we see something that, as of right now, hasn’t yet grown to prominence? Let’s explore this and see what you think!
How Does Our Earliest Picture Of The Universe Show Us Dark Matter?
“So all you need to do, to know whether your Universe has dark matter or not, is to measure these temperature fluctuations that appear in the CMB! The relative heights, locations, and numbers of the peaks that you see are caused by the relative abundances of dark matter, normal matter, and dark energy, as well as the expansion rate of the Universe. Quite importantly, if there is no dark matter, you only see half as many total peaks! When we compare the theoretical models with the observations, there’s an extremely compelling match to a Universe with dark matter, effectively ruling out a Universe without it.”
If your young Universe is full of matter and radiation, what happens? Gravitation works to pull matter into the overdense regions, but that means that the radiation pressure must rise in those regions, too, and that pushes back against the matter. On small scales, this pushback washes out the gravitational growth, but on large-enough scales, the finite speed that light can travel means that no wash-out can happen. Dark matter, however, doesn’t collide with radiation or normal matter, while normal matter collides with both radiation and itself. If we can calculate exactly how these three species interplay, we can calculate what types of patterns we expect to see in the Big Bang’s leftover glow, and then compare it with what we observe with satellites like WMAP and Planck. And what have we seen, exactly, when we’ve done that?
We see that the Universe must contain dark matter to explain the observations. No alternative theory can match it.
Was Our Universe Born In Chaos?
“The model goes through successive intervals of oscillating in two directions while expanding in the third, something like an elevator shaking left and right, back and forth, as it steadily ascends for a number of floors. But then, after a certain number of cycles, called an “era,” one of the oscillating directions swaps places with the expanding direction, and the model begins to grow along a different direction. In the elevator analogy, that would be as if an ascending elevator started to move to the right instead. That transition inaugurates another era, which lasts for a particular number of cycles before switching behavior again to a third direction. Oddly if one writes down the number of cycles for each era, the sequence seems as random as successful dice tosses.”
One of the more puzzling aspects of our Universe is that, no matter which direction we look in, no matter how far away we check, its properties appear to be practically identical. This is surprising, since no signal can reach from one disconnected region to another, and yet the Universe behaves as if everything began from the same initial state. We refer to this as the horizon problem. Before there was cosmic inflation, today’s leading solution to that problem (among others), there was the idea of a Mixmaster Universe, where a combination of oscillations and growth led to a Universe that got smoothed out by the dynamics of its evolution. Although it didn’t solve the horizon problem in details, the chaotic properties of a Mixmaster Universe provided physical and mathematical insights that are still useful today.
Paul Halpern has the historical and scientific facts on this fascinating topic, and simultaneously makes me glad I have a Kitchen Aid instead of a Sunbeam Mixmaster today!
Science Uncovers The Origin Of The First Light In The Universe
“Before there were stars, there was matter and radiation. Before there were neutral atoms, there was an ionized plasma, and when that plasma forms neutral atoms, those allow the Universe to deliver the earliest light we see today. Even before that light, there was a soup of matter and antimatter, which annihilated to produce the majority of today’s photons, but even that wasn’t the very beginning. In the beginning, there was exponentially expanding space, and it was the end of that epoch — the end of cosmic inflation — that gave rise to the matter, antimatter, and radiation that would give rise to the first light we can see in the Universe.”
We normally think of “let there be light” as when the Universe got its start. In a particular sense, this is true, since you can go back to a time before stars and galaxies existed, when the Universe was only a few million years old or less. But even before the first star, there was still light. And while that light’s origin can be traced back to the cosmic microwave background and the formation of neutral atoms, there was light even earlier than that. In fact, the very first light arose from the annihilation of matter and antimatter, and from the end of cosmic inflation at the very beginning of as far back as we can trace.
There was no light until the hot Big Bang occurred, and it’s the end of cosmic inflation that triggers this true “first light.”
Is there really a cosmological constant? Or is dark energy changing with time?
“The Kilo Degree Survey (KiDS) has gathered and analyzed weak lensing data from about 15 million distant galaxies. While their measurements are not sensitive to the expansion of the universe, they are sensitive to the density of dark energy, which affects the way light travels from the galaxies towards us. […]
The members of the KiDS collaboration have tried out which changes to the cosmological standard model work best to ease the tension in the data. Intriguingly, it turns out that ahead of all explanations, the one that works best has the cosmological constant changing with time. The change is such that the effects of accelerated expansion are becoming more pronounced, not less.”
We normally assume that the fundamental constants of the Universe are actually constant, but they don’t have to be that way. They could vary in space, in time, or with the energy density of the Universe, in principle. Before believing in such an extraordinary claim, however, you’d need some remarkable evidence. It’s arguable that exactly that sort of evidence is emerging: from the tensions in the expansion rate of the Universe. If you measure the expansion rate from the cosmic microwave background, you get a value for the expansion rate of 67 km/s/Mpc. But if you measure it from the traditional cosmic distance ladder, you get a value closer to 74. This tension could be a systematic error in the measurement, but it could also point towards the value of dark energy changing with time. Interestingly, a large survey independent of the Universe’s expansion but dependent on weak lensing shows an increasing dark energy might be the answer.
It could all be systematic errors, of course, but if the effect is real, it could revolutionize how we understand the Universe. Sabine Hossenfelder explains.