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.
How Did This Black Hole Get So Big So Fast?
“Recently, a new black hole, J1342+0928, was discovered to originate from 13.1 billion years ago: when the Universe was 690 million years old, just 5% of its current age. It has a mass of 800 million Suns, an exceedingly high figure for such early times. Even if black holes formed from the very first stars, they’d have to accrete matter and grow at the maximum rate possible — the Eddington limit — to reach this size so rapidly. Fortunately, other methods may also grow a supermassive black hole.”
One of the puzzles of how our Universe grew up is how the supermassive black holes we find at the centers of galaxies got so big so fast. We’ve got multiple black holes that come from when the Universe was less than 10% of its current age that are already many hundreds of millions, if not billions, of solar masses in size. How did they get so big so fast? While many hypothesize exotic scenarios like our Universe being born with (primordial) black holes, there is no evidence for such an extraordinary leap. Could conventional astrophysics, and the realistic conditions of our early Universe, actually lead to black holes so massive so early on?
The answer is very likely yes. Come see an extremely favored scenario, with nothing more than conventional astrophysics, that just might get us there.
Ask Ethan: What’s The Real Story Behind This Dark Matter-Free Galaxy?
“I read a study that said the mystery of a galaxy with no dark matter has been solved. But I thought that this anomalous galaxy was previously touted as evidence FOR dark matter? What’s really going on here, Ethan?”
Imagine you looked at the Universe, and saw a galaxy unlike any other. Whereas every other galaxy we’ve ever looked at exhibited a large discrepancy between the amount of matter that’s present in stars and the total amount of gravitational mass we’d infer, this new galaxy appears to have no dark matter at all. What would you do? If you’re being a responsible scientist, you’d try to knock down this galaxy by any scrupulous means possible. You’d wonder if you had mis-estimated one of its properties. You’d try to re-confirm the measurements with different instruments and techniques. And you’d wonder if there weren’t an alternative explanation for what we were seeing.
Well, if you read that the galaxy has dark matter after all, and the mystery has been resolved, you should definitely read this instead. The story is far from over, and even if the new team’s results hold up, there’s still a mystery at play here.
Did Time Have A Beginning?
“Even though we can trace our cosmic history all the way back to the earliest stages of the hot Big Bang, that isn’t enough to answer the question of how (or if) time began. Going even earlier, to the end-stages of cosmic inflation, we can learn how the Big Bang was set up and began, but we have no observable information about what occurred prior to that. The final fraction-of-a-second of inflation is where our knowledge ends.
Thousands of years after we laid out the three major possibilities for how time began — as having always existed, as having begun a finite duration ago in the past, or as being a cyclical entity — we are no closer to a definitive answer. Whether time is finite, infinite, or cyclical is not a question that we have enough information within our observable Universe to answer. Unless we figure out a new way to gain information about this deep, existential question, the answer may forever be beyond the limits of what is knowable.”
If you didn’t know anything about the Universe, you might intuit three possibilities for how time originated. Either it had a beginning a finite duration ago, or it existed for an eternity into the past, or it is cyclical in nature, with no beginning, end, or true delineation between past and future. But we have lots of physical evidence today. We know about the Big Bang and what its limits are. We know about cosmic inflation, which preceded and set up the Big Bang. And we know about dark energy, which determines the fate of our Universe.
With everything we know, what can we say about whether time had a beginning or not? Not enough, unfortunately, but the possibilities remain tantalizing. Find out what we do and don’t know today!
Scientists Discover Space’s Largest Intergalactic Bridge, Solving A Huge Dark Matter Puzzle
“Lo and behold, these shocks are some of the first things you notice if you look at the Chandra images of the Bullet cluster on their own! The fact that we’ve identified relativistic charged particles in the presence of a large-scale magnetic field in one pair of colliding clusters is strongly suggestive of the same effects existing in other clusters. If this same type of structure that exists between Abell 0399 and Abell 0401 also exists between other colliding clusters, it could solve this minor anomaly of the Bullet cluster, leaving dark matter as the sole unchallenged explanation for the displacement of gravitational effects from the presence of normal matter.
It’s always an enormous step forward when we can identify a new phenomenon. But by combining theory, simulations, and the observations of other colliding galaxy clusters, we can push the needle forward when it comes to understanding our Universe as a whole. It’s another spectacular victory for dark matter, and another mystery of the Universe that might finally be solved by modern astrophysics. What a time to be alive.”
When two galaxy clusters collide, the normal matter heats up and emits X-rays, experiencing shocks and separating from the gravitational effects of the clusters they originated from. But as compelling as this evidence is for dark matter, a few of the colliding clusters we’ve found, such as the original (the Bullet cluster), appear to be moving faster than theory predicts. Either dark matter is incomplete, our observations were wrong, or something else is working besides gravity to accelerate the matter.
Guess what? We’ve just found something else between galaxy clusters that accelerates matter: enormous magnetic fields and relativistic electrons! Come get the full story here, and learn what it means and why!
No, This Is Not A Hole In The Universe
“It is absolutely true that billions of light-years away, there are enormous cosmic voids in space. Typically, they can extend for hundreds of millions of light-years in diameter, and a few of them might extend for a billion light-years in size or even many billions of light-years. And one more thing is true: the most extreme ones don’t emit any detectable radiation.
But that is not because there is no matter in them; there is. It’s not because there aren’t stars, gas molecules, or dark matter; all are present. You just can’t measure their presence from emitted radiation; you need other methods and techniques, which show us that these voids still contain substantial quantities of matter. And you definitely shouldn’t confuse them with dark gas clouds and Bok globules, which are small, nearby clouds of light-blocking matter. The Universe is plenty fascinating exactly as it is; let’s resist the temptation to embellish reality with our own exaggerations.”
The Universe, as you may have noticed, is not a uniform place. Just as there are the great cosmic winners, like stars, galaxies, and clusters of galaxies, there are the regions that have to give up their matter to the denser ones. These underdense regions have less normal matter than average, less dark matter, fewer galaxies, stars, and (for the more distant ones) emit no detectable radiation.
Does that mean that they create holes in the Universe? Or that this often-shared image is one of them? Better get the scientific truth today!
Ask Ethan: How Well Has Cosmic Inflation Been Verified?
“To what margin of error or what level of statistical significance would you say you say inflation has been verified?”
So, you’ve got an alternative theory to our best mainstream scientific ideas? Well, guess what: those are the same shoes that every scientific idea we accept today were wearing at one point in the distant past. The thing that separates them from the ideas that fell by the wayside were three remarkable feats:
1. They reproduced all the earlier successes of the previous prevailing model.
2. They resolved or explained puzzles or problems that the previous model had no sufficient answer for.
3. And, perhaps most importantly, they made new predictions that we could go out and test about the Universe, and those predictions were proven correct by the appropriate experimental or observational test.
Although most people don’t appreciate it, inflation has hurdled all three bars, and has no fewer than four spectacular predictions that have since been confirmed. Come learn how well cosmic inflation has been verified today!
When Did The Universe Become Transparent To Light?
“The Universe became transparent to the light left over from the Big Bang when it was roughly 380,000 years old, and remained transparent to long-wavelength light thereafter. But it was only when the Universe reached about half a billion years of age that it became fully transparent to starlight, with some locations experiencing transparency earlier and others experiencing it later.
To probe beyond these limits requires a telescope that goes to longer and longer wavelengths. With any luck, the James Webb Space Telescope will finally open our eyes to the Universe as it was during this in-between era, where it’s transparent to the Big Bang’s glow but not to starlight. When it opens its eyes on the Universe, we may finally learn just how the Universe grew up during these poorly-understood dark ages.”
There are two ways that astrophysicists talk about the Universe becoming transparent. The first is when the particles from the Big Bang finally form neutral atoms, becoming “transparent” to the leftover photons from that era. The second is hundreds of millions of years later, when those same neutral atoms are reionized, and starlight can travel freely through intergalactic space. Which one is right? When did the Universe become transparent to light?
The truth is we need them both, as they make the Universe transparent to different types of light. Come get the full story today.
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.