The Expanding Universe Might Not Depend On How You Measure It, But When
“It’s definitely the case that different methods of measuring the expanding Universe give different values, but this is the first time that the same method has yielded two different results depending on whether you look at the full data set or the late-time measurements alone. The expansion rate of the Universe has been one of the most contentious issues in all of modern science — the Hubble space telescope was even named for its main science goal of measuring that rate, also known as the Hubble constant — and this new result provides a major clue.
Could factoring the effect of cosmic voids in all measurements account for the full discrepancy? Could we be seeing evidence that something, even if it’s not dark energy, is evolving in the Universe in an unexpected fashion? Or, quite possibly, could this be a suggestion that it’s the cosmic microwave background data that’s somehow mistaken, after all? One thing is clear: more and better data, which should be on the way with Euclid, LSST, and WFIRST, will help us decide.”
If you measure the current-day expansion rate of the Universe by looking back and measuring the distances and redshifts to various objects, you get a particular value of around 74 km/s/Mpc. On the other hand, if you look at the earliest light from the Universe, like from the cosmic microwave background, you get a different, inconsistent value of 67 km/s/Mpc. Now, using data from the large-scale structure of the Universe, a new team has discovered something remarkable: if they use all of their data, and factor in the effects of cosmic voids, they get an in-between value of 69 km/s/Mpc. On the other hand, if they restrict themselves to relatively nearby measurements, they get a much higher value: closer to 72.3 km/s/Mpc.
What, exactly, is going on? This is a presently unsolved puzzle in cosmology, but this new study reveals one more clue that brings us closer to understanding what’s happening.
Ask Ethan: How Can We See 46.1 Billion Light-Years Away In A 13.8 Billion Year Old Universe?
“If the limit of what we could see in a 13.8 billion year old Universe were truly 13.8 billion light-years, it would be extraordinary evidence that both General Relativity was wrong and that objects could not move from one location to a more distant location in the Universe over time. The observational evidence overwhelming indicates that objects do move, that General Relativity is correct, and that the Universe is expanding and dominated by a mix of dark matter and dark energy.
When you take the full suite of what’s known into account, we discover a Universe that began with a hot Big Bang some 13.8 billion years ago, has been expanding ever since, and whose most distant light can come to us from an object presently located 46.1 billion light-years away. The space between ourselves and the distant, unbound objects we observe continues to expand at a rate of 6.5 light-years per year at the most distant cosmic frontier. As time goes on, the distant reaches of the Universe will further recede from our grasp.”
The fabric of space is expanding, and this is perhaps the most counterintuitive thing of all. Back in the earliest stages of the Big Bang, a point in space no farther away from us than the length of a city block could have emitted a photon, and it would take that photon a remarkable 13.8 billion years just to reach us. Moreover, that photon would journey for a total of 13.8 billion light-years before arriving at our eyes, and an object located at the exact same point in space where it was initially emitted from would now be 46.1 billion light-years away from us.
Why is this? Because that’s what the laws of General Relativity, coupled with our knowledge of what’s present in the Universe, demand of space and time. Come get the full story today!
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!