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.
Scientists Can’t Agree On The Expanding Universe
“The question of how quickly the Universe is expanding is one that has troubled astronomers and astrophysicists since we first expansion was occurring at all. It’s an incredible achievement that multiple, independent methods yield answers that are consistent to within 10%, but they don’t agree with each other, and that’s troubling.
If there’s an error in parallax, Cepheids, or supernovae, the expansion rate may truly be on the low end: 67 km/s/Mpc. If so, the Universe will fall into line when we identify our mistake. But if the Cosmic Microwave Background group is mistaken, and the expansion rate is closer to 73 km/s/Mpc, it foretells a crisis in modern cosmology. The Universe cannot have the dark matter density and initial fluctuations 73 km/s/Mpc would imply.
Either one team has made an unidentified mistake, or our conception of the Universe needs a revolution. I’m betting on the former.”
The Universe is expanding: the observations overwhelmingly support that. It’s consistent with Einstein’s General Relativity; it work with the framework of the Big Bang; it allows us to quantify and predict the ultimate fate of our Universe.
But how fast, then, is the Universe expanding?
Scientists can’t agree, because there are three different techniques you can use to measure it. Two agree; one doesn’t.
So what gives? This is the controversy driving astrophysicists nuts at the moment. Come learn what it’s all about, along with my hunch as to what the resolution will be!
Who Really Discovered The Expanding Universe?
“Recently, what was known for generations as “Hubble’s Law” has now been renamed the Hubble-Lemaître law. But the point shouldn’t be to give credit to individuals who’ve been dead for generations, but rather for everyone to understand how we know the rules that govern the Universe, and what they are. I, for one, would be just as happy to drop all the names from all the physical laws out there, and simply to refer to them as what they are: the redshift-distance relation. It wasn’t the work of just one or two people that led to this breakthrough in discovering the expanding Universe, but of all the scientists I named here and many others as well. At the end of the day, it’s our fundamental knowledge of how the Universe works that matters, and that’s the ultimate legacy of scientific research. Everything else is just a testament to the all-too-human foible of vainly grasping at glory.”
In science, we have a tendency to name theories, laws, equations, or discoveries after the individual who made the greatest contribution towards its development. For generations, we credited Edwin Hubble for discovering the expanding Universe, as his contributions in the 1920s were absolutely tremendous. However, history has not only revealed that Georges
discovered the very law we had named after Hubble two years prior, but that many other people made essential contributions to that realization. The expanding Universe didn’t come about solely because of Hubble’s discoveries, and perhaps we can do better than crediting just a single person.
Here are a slew of advances that led to and supported the expanding Universe, showing that history and science relies on contributions far richer than that of a lone, genius scientist.
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.
Surprise! The Hubble Constant Changes Over Time
“If astronomers were more careful about their words, they would have called H the Hubble parameter, rather than the Hubble constant, since it changes over time. But for generations, the only distances we could measure were close enough that H appeared to be constant, and we’ve never updated this. Instead, we have to be careful to note that H is a function of time, and only today — where we call it H0 — is it a constant. In reality, the Hubble parameter changes over time, and it’s only a constant everywhere in space. Yet if we lived far enough in the future, we’d see that H stops changing entirely. As careful as we can be to make the distinction between what’s actually constant and what changes now, in the far future, dark energy ensures there will be no difference at all.”
The farther away you look in space, the more redshifted the light from the object you’re viewing appears. This implies that the Universe is expanding, and the rate of expansion, known as the Hubble constant, is something we’ve strived to measure for a very long time. While there’s a minor controversy over just what that expansion rate is today, whether it’s 67 or 73 km/s/Mpc, it’s perhaps surprising to learn that the expansion rate has changed over time. That means that the Hubble constant isn’t actually a constant at all! So why do we call it that? Because we’re evaluating how the Universe is expanding today. It’s constant everywhere in space, but has actually dropped. Interestingly enough, it will now asymptote to a finite, unchanging value. Many billions of years in the future, it will truly become a constant.
The fact that the energy density has changed over time means that the Hubble expansion rate has changed, too, and that the Hubble constant actually changes over time! Come find out how.
Ask Ethan: How Many Galaxies Have Already Disappeared From Our Perspective?
“So how many earth observable galaxies have dropped out of sight? That is, how many galaxies (with the highest redshift) have disappeared from our point of view?”
When we look out at the distant reaches of space, there are some 2 trillion galaxies observable within our Universe. But our Universe is expanding, the expansion is accelerating, and light can only travel at the speed of light. Does that mean that galaxies are dropping out of sight?
There are two ways to look at this: from the point of view of not being able to see galaxies that we can presently see, and from the point of view of whether we can see the light those galaxies are emitting today, 13.8 billion years after the Big Bang? If we take the first definition, not only is the answer “zero,” but there will be trillions more galaxies revealed to us over time. But if we take the second, we find that most of the galaxies we can see today are already gone.
How many galaxies have already disappeared from our perspective? The cosmic implications should motivate us to get out there and explore while there’s still some good Universe left to go and see!
How Come Cosmic Inflation Doesn’t Break The Speed Of Light?
“In an inflationary Universe, any two particles, beyond a tiny fraction of a second, will see the other one recede from them at speeds appearing to be faster-than-light. But the reason for this isn’t because the particles themselves are moving, but rather because the space between them is expanding. Once the particles are no longer at the same location in both space and time, they can start to experience the general relativistic effects of an expanding Universe, which — during inflation — quickly dominates the special relativistic effects of their individual motions. It’s only when we forget about general relativity and the expansion of space, and instead attribute the entirety of a distant particle’s motion to special relativity, that we trick ourselves into believing it travels faster-than-light. The Universe itself, however, is not static. Realizing that is easy. Understanding how that works is the hard part.”
It’s true that nothing can move faster than the cosmic speed limit, the speed of light, and that no two particles can move faster than light relative to one another. So how, then, do you explain the fact that during inflation, two particles that begin a subatomic distance away from one another are, after just a tiny fraction of a second, are then billions of light years apart? The answer is because special relativity only applies, strictly, to particles that occupy the same location as one another in both space and time. If they’re separated, then the Universe is under no obligation to be static, and space is free to expand and/or contract. You cannot figure your apparent motion with special relativity alone, but need to factor in the effects of general relativity as well. And that’s where things get really weird.
If you can understand it, however, the notion of how objects appear to recede faster than light suddenly starts to make sense. Come learn how inflation doesn’t break the speed of light after all!