This Is How Astronomers Will Finally Measure The Universe’s Expansion Directly
“This is why, by measuring the redshifts and distances to a slew of objects — objects at a variety of different distances and redshifts — we can reconstruct the expansion of the Universe over its history. The fact that a whole slew of disparate data sets are all consistent with not only one another but with an expanding, evenly filled Universe in the context of relativity, that gives us the confidence we have in our model of the Universe.
But, just as we didn’t necessarily accept gravitational waves before they were directly measured by LIGO, there’s still the possibility that we’ve made a mistake somewhere in inferring the properties of the Universe. If we could take a distant object, measure its redshift and distance, and then come back at a later time to see how its redshift and distance had changed, we’d be able to directly (instead of indirectly) measure the expanding Universe for the first time.”
We’ve measured the distance to literally billions of objects all over the Universe, from within our galaxy to more than 30 billion light-years away. By observing how the light from these distant objects is shifted, we’re able to infer that the Universe is expanding. We’re able to infer how that expansion rate has changed over time. And we’re able to infer what the Universe is made of: a monumental accomplishment.
But what we’ve never been able to do, as of 2019, is to watch an individual, distant galaxy physically expand away from us in real-time. With the new generation of 30-meter class telescopes we have coming online, though, all of that is poised to change. When the ELT arrives, the largest of the next generation telescopes at 39 meters, it will have the capability to make this measurement directly by observing the same sets of quasars 10 years apart.
You’re going to get to learn a new term today: redshift drift. When we measure it, we’ll have our first direct observation of the Universe as it physically expands on human timescales.
This Is How Distant Galaxies Recede Away From Us At Faster-Than-Light Speeds
“All the galaxies in the Universe beyond a certain distance appear to recede from us at speeds faster than light. Even if we emitted a photon today, at the speed of light, it will never reach any galaxies beyond that specific distance. It means any events that occur today in those galaxies will not ever be observable by us. However, it’s not because the galaxies themselves move faster than light, but rather because the fabric of space itself is expanding.
In the 7 minutes it took you to read this article, the Universe has expanded sufficiently so that another 15,000,000 stars have crossed that critical distance threshold, becoming forever unreachable. They only appear to move faster than light if we insist on a purely special relativistic explanation of redshift, a foolish path to take in an era where general relativity is well-confirmed. But it leads to an even more uncomfortable conclusion: of the 2 trillion galaxies contained within our observable Universe, only 3% of them are presently reachable, even at the speed of light.
If we care to explore the maximum amount of Universe possible, we cannot afford to delay. With each passing moment, another chance for encountering intelligent life forever slips beyond our grasp.”
If you look at a galaxy, chances are you’ll see that it appears to be receding away from us, as its light is redshifted. The more distant you look, the greater the redshift, and hence, the faster the implied recession speed. But this interpretation runs into problems very quickly: by the time you’re looking at galaxies more than 13-to-15 billion light-years away, they start to appear as though they’re receding faster than the speed of light!
Impossible, you say? Sure, if you only consider special relativity. If you insist on general relativity, it all falls into place. Here’s how.
This Is Why Einstein’s Greatest Blunder Really Was A Tremendous Mistake
“But there’s no retconning history; Einstein wasn’t right after all. While our Universe might actually have a non-zero cosmological constant, it isn’t there to stabilize our Universe. Rather, our Universe isn’t stable at all; it’s expanding from an initially hot, dense, and uniform state into the cold, sparse, and galaxy-rich cosmos we see today.
Einstein missed all of that because he insisted on a static Universe, and invented the cosmological constant to achieve that goal. Take it away, and you get a Universe that’s very much like the one we have today. The cosmological constant that affects our Universe serves to break the balance between the expansion and the other forms of matter-and-energy; it causes distant galaxies to accelerate away from us, pushing the Universe apart. Had Einstein predicted that, it would have been mind-boggling. Instead, he forced the equations to fit his (incorrect) assumptions, and missed the expanding Universe.”
When Einstein first set forth his General theory of Relativity, it included a term that no one had ever heard of before: a cosmological constant. Einstein had realized that a static Universe, the one he thought he lived in, was unstable. Gravitation would cause matter to collapse, and so something had to counteract that. His solution was to concoct a cosmological constant, something that he called his “greatest blunder” after the expanding Universe was confirmed.
Does the late-1990s discovery of dark energy, which might be a cosmological constant after all, mean that Einstein was actually right? Not at all. Come find out why today.
Astronomically Rare ‘Double Lens’ Yields Best Single System Measurement Of Cosmic Expansion
“Methods based on early signals imprinted in the cosmic microwave background and on the Universe’s large-scale structure indicate one value: 67 km/s/Mpc. However, methods relying on precise measurements to distant objects deliver a conflicting value: 74 km/s/Mpc. With overall errors of just 1-2% apiece, this 9% difference is significant and robust. Each new measurement has the opportunity to either validate or refute this growing tension.”
How quickly is the Universe expanding? You might think that’s a simple question, and it would be if every way we had of measuring that rate gave the same consistent answer. Only, what we’re finding is something very strange: measuring the expansion rate using an early-time signal gives one value, and measuring it using a late-time signal gives a different, inconsistent value.
The next step is to come up with as many different methods as possible of measuring this rate, and to see if the discrepancy persists. In a novel 2017 find, the system DES J0408-5354 was discovered, appearing to be a background objects lensed four times by a foreground galaxy. As it turns out, though, this is actually a double lens: two independent background sources lensed by the same foreground source, with each one creating multiple images.
This is an unprecedented system for measuring the expansion rate, and yields a value with just 3.9% uncertainty. Which group did it agree with? Come find out as the mystery deepens today!
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.