The Expanding Universe Puzzle Just Got Worse, As Incompatible Answers Point To New Physics
“Could there be a problem with our local density relative to the overall cosmic density? Could dark energy change over time? Could neutrinos have an additional coupling we don’t know about? Could the cosmic acoustic scale be different than the CMB data indicates? Unless some new, unexpected source of error is uncovered, these will be the questions that drive our understanding of the Universe’s expansion forward. It’s time to look beyond the mundane and seriously consider the more fantastic possibilities. At last, the data is strong enough to compel us.”
You’ve heard this before, commonly referred to as the “tension” in the expansion rate of the Universe. Two sets of groups are obtaining different values for how fast the Universe is expanding, and the value they get is either close to 67 km/s/Mpc (if you use an early Universe signal) or 73 km/s/Mpc (if you use a late Universe signal). A new result published this week in Science bolsters this, but a reanalysis of the one late Universe signal with a low value (of 69.8 km/s/Mpc) is the biggest deal, as improved calibrations bump that number up by ~4%, enough to put it in line with the other late Universe signals.
If neither the early nor the late group has made a mistake, the true answer is unlikely to lie in the middle. This is why, and here’s what, as a field, astrophysicists need to do about it.
If Cosmology Is In Crisis, Then These Are The 19 Most Important Galaxies In The Universe
“In science, different methods of measuring the same properties should yield the same results. But when it comes to the expanding Universe, two sets of groups get consistently different outcomes. Signals from the early Universe yield expansion rates of 67 km/s/Mpc, while late-time signals yield systematically larger values. However, every individual measurement is subject to errors and uncertainties inherent to the method used.”
The strength of any method used in a scientific practice is only as good as the weakest assumption or measurement that’s made. In the case of measuring the expanding Universe, astronomers using an early-time signal get results that are systematically 9% smaller than astronomers using a late-time signal. Of all the late-time signals, the one method with the smallest uncertainties relies on the cosmic distance ladder: tying parallax measurements to Cepheids in the Milky Way, then tying Cepheids to galaxies with Type Ia supernovae, then measuring supernovae everywhere in the Universe. However, there are only 19 galaxies where Type Ia supernovae have been observed that are close enough to have observed Cepheids within them. A tiny statistical fluctuation in the properties of these galaxies could be enough to resolve most or even all of this discrepancy.
It may not be the most likely outcome, but it’s something to keep an eye on. If cosmology is in crisis, then these may be the 19 most important galaxies of all.
Ask Ethan: Can We Really Get A Universe From Nothing?
“One concept bothers me. Perhaps you can help. I see it in used many places, but never really explained. “A universe from Nothing” and the concept of negative gravity. As I learned my Newtonian physics, you could put the zero point of the gravitational potential anywhere, only differences mattered. However Newtonian physics never deals with situations where matter is created… Can you help solidify this for me, preferably on [a] conceptual level, maybe with a little calculation detail?”
You’ve very likely heard two counterintuitive things about the Universe before. One of them is that the Universe arose from nothing, and this defies our intuition about how it’s impossible to get something from nothing. The second is that we have four fundamental forces in the Universe: gravity, electromagnetism, and the strong and weak nuclear forces. So how, then, do we account for the fact that the Universe’s expansion is accelerating? Isn’t this clearly evidence for a fifth force, one with negative gravity?
Guess what? These two counterintuitive aspects of reality are related. If you understand them both, you’re one step closer to making sense of the Universe.
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!
No, The Universe Cannot Be A Billion Years Younger Than We Think
“There may be some who contend we don’t know what the age of the Universe is, and that this conundrum over the expanding Universe could result in a Universe much younger than what we have today. But that would invalidate a large amount of robust data we already have and accept; a far more likely resolution is that the dark matter and dark energy densities are different than we previously suspected.
Something interesting is surely going on with the Universe to provide us with such a fantastic discrepancy. Why does the Universe seem to care which technique we use to measure the expansion rate? Is dark energy or some other cosmic property changing over time? Is there a new field or force? Does gravity behave differently on cosmic scales than expected? More and better data will help us find out, but a significantly younger Universe is unlikely to be the answer.”
There’s a fascinating conundrum facing modern cosmology today. If you measure the distant light from the Universe, from the cosmic microwave background or from how the large-scale structure within it has evolved, you can get a value for the expansion rate of the Universe: 67 km/s/Mpc. On the other hand, you can also get a measurement for that rate from measuring individual objects through a technique known as the cosmic distance ladder, and you get a value of 73 km/s/Mpc. These two values differ by 9%, and are inconsistent with one another. Recently, one of the groups studying this puzzle claimed that the Universe might be 9% younger than currently expected: 12.5 billion years old instead of 13.8 billion years old.
That is almost certainly wrong, as it would conflict with extremely important pieces of astronomical data. This really is a puzzle, but a younger Universe isn’t the solution. Here’s why.
Cosmology’s Biggest Conundrum Is A Clue, Not A Controversy
“This is not some fringe idea, where a few contrarian scientists are overemphasizing a small difference in the data. If both groups are correct — and no one can find a flaw in what either one has done — it might be the first clue we have in taking our next great leap in understanding the Universe. Nobel Laureate Adam Riess, perhaps the most prominent figure presently researching the cosmic distance ladder, was kind enough to record a podcast with me, discussing exactly what all of this might mean for the future of cosmology.
It’s possible that somewhere along the way, we have made a mistake somewhere. It’s possible that when we identify it, everything will fall into place just as it should, and there won’t be a controversy or a conundrum any longer. But it’s also possible that the mistake lies in our assumptions about the simplicity of the Universe, and that this discrepancy will pave the way to a deeper understanding of our fundamental cosmic truths.”
In science, if you want to know some property of the Universe, you need to devise a measurement or set of measurements you can make to reveal the quantitative answer. When it comes to the expanding Universe, we have many different methods of measuring light that fall into two independent classes: using the imprint of an early relic and using the cosmic distance ladder. These two techniques each give solid results that are mutually inconsistent: the distance ladder teams find results that are higher than the early relic teams by about 9%. Since the errors are only about 1-2% on each measurements, this has been dubbed cosmology’s biggest controversy.
But perhaps it’s not about “who is right,” but rather about “what is the Universe doing?” Perhaps it’s a clue, not a controversy. Come learn about the cutting-edge science behind this fascinating and unexpected result.
Ask Ethan: Could ‘Cosmic Redshift’ Be Caused By Galactic Motion, Rather Than Expanding Space?
“When we observe a distant galaxy, the light coming from the galaxy is redshifted either due to expansion of space or actually the galaxy is moving away from us. How do we differentiate between the cosmological redshift and Doppler redshift? I have searched the internet for answers but could not get any reasonable answer.”
It’s true: the farther away we look, the greater we find a galaxy’s redshift to be. But why is that? You may have heard the (correct) answer: because space is expanding. But how do we know that? Couldn’t something else be causing this redshift?
The answer is yes, there are actually four other explanations for cosmic redshift that all make sense. But the beauty of science is that there are observational tests we can perform to tell these various scenarios apart! We’ve done those tests, of course, and concluded the Universe is expanding, but wouldn’t you like to know how?
I bet you would! Come and find out how we know that cosmic redshift is caused by the expansion of the Universe, and learn where the alternatives fall apart.
How Much Of The Unobservable Universe Will We Someday Be Able To See?
“You might think that if we waited for an arbitrarily long amount of time, we’d be able to see an arbitrarily far distance, and that there would be no limit to how much of the Universe would become visible.
But in a Universe with dark energy, that simply isn’t the case. As the Universe ages, the expansion rate doesn’t drop to lower and lower values, approaching zero. Instead, there remains a finite and important amount of energy intrinsic to the fabric of space itself. As time goes on in a Universe with dark energy, the more distant objects will appear to recede from our perspective faster and faster. Although there’s still more Universe out there to discover, there’s a limit to how much of it will ever become observable to us.”
The Universe is a huge, vast, enormous place. It’s been 13.8 billion years since the Big Bang occurred, which translates into an observable Universe that’s 46 billion light years to its edge, and contains some 2 trillion galaxies in various stages of evolutionary development. But that’s not the end of what we’ll ever be able to observe. As time goes on, light that’s presently on its way to our eyes will eventually catch up, revealing a future visibility limit that’s even larger than the present observable Universe. When we add it all up, we’ll find that we more than double the number of galaxies we can observe, even though we can barely reach 1% of them.
How does this all work? Find out the limits of the observable and unobservable Universe today!
How Did The Universe Expand To 46 Billion Light-Years In Just 13.8 Billion Years?
“The fact that we can see the Universe we do tells us that it must be expanding, a fantastic match of theory and observation. It also tells us that we can extrapolate back in time to as early a stage as we want, and find all sorts of interesting milestones that happen as far as the size of the Universe is concerned compared with its age. When the Universe was a million years old, its edge was already some 100 million light-years away. When it was just a year old, we could see for nearly 100,000 light-years. When it was just a millisecond old, we could already see for a light-year in all directions.
And today, 13.8 billion years after the Big Bang, the farthest thing we could possibly see, corresponding to the light emitted at the first moment of the Big Bang, is 46.1 billion light-years distant. Given the contents of our Universe, it couldn’t have turned out any other way.”
When you start telling people about the expanding Universe, one fact always puzzles them. The Universe itself has been around for 13.8 billion years since the Big Bang occurred. There’s nothing at all that can travel faster than the speed of light; that speed is an unbreakable cosmic speed limit. So how, then, is the farthest objects we can see 46 billion light-years away today? How did the observable Universe get bigger than its age seems to allow?
The answer isn’t that the Universe expanded faster than light. It’s merely that the Universe has been expanding, and that doesn’t mean what you might first think!
This Is Why We Aren’t Expanding, Even If The Universe Is
“As long as the Universe has the properties we measure it to have, this will remain the case forever. Dark energy may exist and cause the distant galaxies to accelerate away from us, but the effect of the expansion across a fixed distance will never increase. Only in the case of a cosmic “Big Rip” — which the evidence points away from, not towards — will this conclusion change.
The fabric of space itself may still be expanding everywhere, but it doesn’t have a measurable effect on every object. If some force binds you together strongly enough, the expanding Universe will have no effect on you. It’s only on the largest scales of all, where all the binding forces between objects are too weak to defeat the speedy Hubble rate, that expansion occurs at all. As physicist Richard Price once put it, “Your waistline may be spreading, but you can’t blame it on the expansion of the universe."”
On the largest cosmic scales, everywhere we look, we see things moving away from us. The distant galaxies are receding not only from our perspective, but from one another. The Universe is expanding, a scientific fact that’s now nearly 100 years old. But we ourselves aren’t. Atoms remain the same size, as do our bodies, as do the scales of planets, solar systems, stars, and individual galaxies. Even groups and clusters of galaxies don’t appear to expand.
Why is that? Why aren’t we expanding, even as the Universe itself expands? Come get the physical explanation of the most profound phenomenon in the Universe.