How Can We Still See The Disappearing Universe?
“In fact, we can even think about what you’d see if you were to look at a galaxy whose light hasn’t arrived at our eyes yet. The most distant object we can see, 13.8 billion years after the Big Bang, is presently 46 billion light-years away from us. But any object that’s presently within 61 billion light-years of us will someday have that light eventually reach us.
That light was already emitted, and is already on its way to us. In fact, that light is already most of the way there; it’s closer than the 15 billion light-year limit of what we could possibly reach if we left for it at the speed of light. Even though the Universe is expanding, and even though the expansion is accelerating, that journeying light will someday arrive at our eyes, giving us, in the far future, the ability to see even more galaxies than we can today.”
Dark energy seems to present a paradox. On the one hand, galaxies are receding from us as the Universe expands, meaning we can never reach them once they’re beyond a certain point, and that the light being emitted by them can also no longer reach us. But even though these galaxies are a part of our dark energy-dominated Universe, we’ll always be able to see them in the future once they become visible to us.
If the Universe is disappearing, how can we still see the galaxies in it? Come get the answer to one of cosmology’s biggest (misconception-related) puzzles today!
Ask Ethan: If The Universe Ends In A Big Crunch, Will All Of Space Recollapse?
“When you describe the Big Crunch, you talk about a race between gravity and the expansion of space. It’s not clear to me that if gravity wins that race, whether space stops expanding, or simply that the matter in space stops expanding. I’d love to hear your explanation of this.”
The Universe is expanding, and we can confirm this by looking at the relationship between how redshifted a galaxy’s light is compared with how far away it is from us. But if these galaxies, at some point in the far future, stop being redshifted and start moving closer and closer to us again, does that necessarily mean that the fabric of space is contracting? Is all of space necessarily recollapsing? Or could the galaxies simply be moving towards us, owing to some massive attraction, while the fabric of space doesn’t recollapse at all? Does a Big Crunch necessarily equate to a recollapsing Universe?
Even though we don’t know whether dark energy will reverse itself or not, we do know the answer to this question, and yes, a Big Crunch does mean recollapse! Find out why on this edition of Ask Ethan.
What Was It Like When Dark Energy First Took Over The Universe?
“In reality, we can only make observations at one point in time: today, or when the light from all the distant objects throughout the Universe is finally reaching us. But we can imagine our hypothetical scenario just as well.
What would we see if we could track a single, individual galaxy — including both its distance and its redshift as seen from our perspective — throughout the history of the Universe?
The answer may be a little counterintuitive, but it’s tremendously illustrative and educational as far as shedding light on not only what dark energy is, but how it affects the expansion of the Universe.”
If you could put your finger down on a distant, individual galaxy far away and unbound from our own, what would you see if you could track its motion over time? For the first few billion years, it would be moving away very quickly, getting more and more distant, but it would appear to slow down. It would be as though gravity were trying to pull it back to us, albeit unsuccessfully. And then, at a critical moment some 7.8 billion years after the Big Bang, this slowing down would cease. The galaxy would transition from decelerating to accelerating, and would speed away from us, faster and faster, ever after.
This marks the transition to when dark energy took over the expanding Universe. Come find out what it was like when that happened, and how, today!
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.
The Five Ways The Universe Might End
“4.) Dark energy could transition into another form of energy, rejuvenating the Universe. If dark energy doesn’t decay, but instead remains constant or even strengthens, there’s another possibility that arises. This energy, inherent to the fabric of space today, may not remain in that form forever. Instead, it could get converted into matter-and-radiation, similar to what occurred when cosmic inflation ended and the hot Big Bang began.
If dark energy remains constant until that point, it will create a very, very cold and diffuse version of the hot Big Bang, where only neutrinos and photons can self-create. But if dark energy increases in strength, it could lead to an inflation-like state followed by a new, truly hot Big Bang once again. This is the most straightforward way to rejuvenate the Universe, and create a cyclic-like set of parameters, where the Universe gets another chance to behave like ours did.”
Based on the best knowledge and data that we have today, it’s clear that the Universe isn’t just expansion, but the expansion is accelerating. Does this determine the fate of our Universe unambiguously? If we extrapolate what the data indicates about dark energy into the future, we fully expect that structures (like our local group) that are gravitationally bound today will remain so into the future, but that larger-scale structures which are unbound (like our supercluster, Laniakea) will eventually dissociate. But extrapolation is tricky, and assumes that dark energy doesn’t change over time.
If we allow the possibility of change, though, many more possibilities arise. Here are the five most likely, and how we’ll distinguish between them!
This Is How Dark Energy’s Main Competitor Failed
“The reason is simple: with the addition of enough extra free parameters, caveats, behaviors, or modifications to your theory, you can literally salvage any idea. As long as you’re willing to tweak what you’ve come up with sufficiently, you can never rule anything out. If you wanted to concoct a dusty explanation that mimicked the effects of dark energy, you could do it. At some point, though, you lose all physical motivation, and you’re coming up with multi-parameter explanations to explain an observation that a single free parameter — dark energy — gave you before you started tinkering with your dust theory.”
When we look out at the ultra-distant Universe, Type Ia supernovae are our most distant standard candle to work with. From billions or even tens of billions of light years away, we think we know the intrinsic brightnesses of these objects. So measure the apparent brightness, and you know how far away they are, right?
Well, not so fast. What if there’s dust or some other light-blocking phenomenon intervening? Could that mean that these objects are closer than we think, and therefore there’s no need for dark energy? It’s a great idea, and one that we investigated for many years, until the data convincingly showed that no, dust cannot work.
Want to find out why dark energy is real, and this isn’t due to the effects of dust? Have a look today!
These Are The Last Galaxies That Will Remain In Our Night Sky
“But beyond our backyard, all the other galaxies, groups, and clusters are accelerating away from us.
Once you go about 4-5 million light years away, dark energy causes space to expand faster than gravity attracts other objects across space.
Over time, every other galaxy will see its distance and recession speed increase from our perspective.”
Our Universe isn’t only expanding, but is accelerating. This means that every galaxy, group, or cluster that isn’t already gravitationally bound to us is receding from us at a faster and faster rate as time goes on. With 2 trillion galaxies in the observable Universe, only approximately 70 of them are bound to our Local Group. Everything farther than about 4 or 5 million light years away from us is unbound, and therefore will recede from us forever and ever, with their recession speeds and cosmic distances increasing over time.
Here are the last galaxies we’ll ever be able to see or visit, as the relentless expansion of the Universe causes their disappearance from our cosmic horizon.
This Is Why Dark Energy Must Exist, Despite Recent Reports To The Contrary
“We do not do science in a vacuum, completely ignoring all the other pieces of evidence that our scientific foundation builds upon. We use the information we have and know about the Universe to draw the best, most robust conclusions we have. It is not important that your data meet a certain arbitrary standard on its own, but rather that your data can demonstrate which conclusions are inescapable given our Universe as it actually is.
Our Universe contains matter, is at least close to spatially flat, and has supernovae that allow us to determine how it’s expanding. When we put that picture together, a dark energy-dominated Universe is inescapable. Just remember to look at the whole picture, or you might miss out on how amazing it truly is.”
20 years ago, the supernova data came back with an extraordinary surprise: it looked like the Universe wasn’t just expanding, but that the expansion rate was increasing as we head further into the future. While there were many dark energy skeptics to start, the increased flow of improved data from many lines of evidence that all kept pointing to the same conclusion has led to a cosmological consensus: dark energy dominates the Universe today. Last week, a story made waves, as Subir Sarkar and collaborators published their second paper (the first was in 2016) claiming that the evidence from supernovae is not good enough to support the existence of dark energy, and our cosmological foundation for it is extraordinarily shaky.
This is not true. This is demonstrably untrue. And the claim shows a deliberate unwillingness to pay attention to the rest of the field. Find out why dark energy must exist, despite recent reports to the contrary.
Ask Ethan: When Were Dark Matter And Dark Energy Created?
“Today [normal matter] is only 4.9% while Dark Matter and Dark Energy takes the rest. Where did they come from?”
The Universe, as we know it, got its start in earnest when the hot Big Bang began. Space was filled with all the particles and antiparticles of the Standard Model, up at tremendous energies, while the Universe then expanded, cooled, and gave rise to all we know. But when did dark matter and dark energy, which make up 95% of the Universe we know today, come into the picture? Was the Universe born with these components of energy? Or were they created at a later time? We have some inklings that dark matter was likely created in the extremely early stages, but may not have been present from the Universe’s birth. On the other hand, all theoretical signs point to dark energy always existing, but observationally, we have about 4 billion years where we cannot measure its presence at all.
Where do dark matter and dark energy come from? It’s a great cosmic mystery, but we do know something about it. Find out where we are today!
The Universe Is Disappearing, And There’s Nothing We Can Do To Stop It
“Of the estimated two trillion galaxies in our Universe today, only about 3% of them are still reachable from the point of view of the Milky Way. This also means that 97% of the galaxies in our observable Universe are already out of humanity’s reach, owing to the accelerated expansion of the Universe caused by dark energy. Every galaxy beyond our local group, as time goes on, is destined for that same fate.
Unless we develop the capacity for intergalactic travel and head out to other galaxy groups and clusters, humanity will forever be stuck in our local group. As time goes on, our ability to even send or receive signals to what lies beyond in the great cosmic ocean will fade from view. The accelerated expansion of the Universe is relentless, and the gravity we have isn’t strong enough to overcome it. The Universe is disappearing, and there’s nothing we can do to stop it.”
One of the most profound discoveries about the Universe occurred just 20 years ago. Not only was the Universe expanding, with distant galaxies getting farther and farther away as time goes on, but that expansion was accelerating. Take a look at any galaxy that isn’t gravitationally bound to our own, and if you watch it as time goes on, it will appear to move away from us faster and faster. At some point, when it gets to about 15 billion light years away from us, it will appear to recede faster than the speed of light. When it reaches that point, it means that anything that occurs within it won’t be viewable by us, and that if we left immediately, even at the speed of light, we’d never reach it.
Moreover, with every second that goes by, approximately 20,000 new stars cross that threshold into unreachability. The Universe is disappearing, and there’s nothing we can do about it.