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.
How The Planck Satellite Forever Changed Our View Of The Universe
“Most importantly, a spectacular agreement to a never-before-achieved precision now exists between the CMB we observe and the theoretical predictions of a Universe with 5% normal matter, 27% dark matter, and 68% dark energy. There might be wiggle room of 1-2% in some of those numbers, but a Universe without dark matter and dark energy, both, in great abundance, is a no-go in the face of these observations. They’re real, they’re necessary, and their predictions match the full suite of data perfectly.
Inflation, neutrino physics, and the Big Bang have additional pieces confirming them, while alternatives and specific variants are better constrained. Most definitively, the Planck collaboration states, “We find no compelling evidence for extensions to the base-ΛCDM model.” At last, we can state, with extraordinary confidence, what the Universe is made of.”
For centuries, the question of what the Universe was made of was one of the most unknowable wonders of existence. This week, the Planck collaboration, whose team made the most accurate, precision measurements of the Big Bang’s leftover glow, released their final results, providing unprecedented answers to that question. We now know what the Universe is made of, how old it is, how fast it’s expanding, and a whole suite of other information about it, better than we’ve ever known before. The Planck satellite has revolutionized our view of the Universe, and we’re unlikely to ever do better using this line of inquiry.
How has our view of the Universe changed as a result of Planck? Come find out today!
Ask Ethan: Could The Universe Be Torn Apart In A Big Rip?
“Is The Big Rip—where expansion exceeds all the other forces—still considered a possible future for our Universe? What are the arguments for or against? And if so, how would it unfold, what would happen?”
In addition to normal matter, dark matter, neutrinos, and radiation, the Universe is made up of dark energy: a new form of energy intrinsic to space itself. Although the data indicates that dark energy is consistent with being a cosmological constant, whose energy density won’t change with time, it’s possible that this energy will increase or decrease in strength. If it decreases, it could decay entirely or even reverse sign. resulting in a Big Crunch. But if it increases, we could have a spectacularly catastrophic fate: the Big Rip. In the Big Rip, bound objects will literally be ripped apart on galactic, stellar, planetary, and eventually even atomic scales. Even space itself will rip apart in the end.
The Big Rip isn’t ruled out, but if it’s going to occur, our current constraints push it out to 80 billion years in the future. Find out what it would look like and how we’ll know!
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: Could The Energy Loss From Radiating Stars Explain Dark Energy?
“What happens to the gravity produced by the mass that is lost, when it’s converted by nuclear reactions in stars and goes out as light and neutrinos, or when mass accretes into a black hole, or when it’s converted into gravitational waves? […] In other words, are the gravitational waves and EM waves and neutrinos now a source of gravitation that exactly matches the prior mass that was converted, or not?”
For the first time in the history of Ask Ethan, I have a question from a Nobel Prize-winning scientist! John Mather, whose work on the Cosmic Microwave Background co-won him a Nobel Prize with George Smoot, sent me a theory claiming that when matter gets converted into radiation, it can generate an anti-gravitational force that might be responsible for what we presently call dark energy. It’s an interesting idea, but there are some compelling reasons why this shouldn’t work. We know how matter and radiation and dark energy all behave in the Universe, and converting one into another should have very straightforward consequences. When we take a close look at what they did, we can even figure out how the theory’s proponents fooled themselves.
Radiating stars and merging black holes do change how the Universe evolves, but not in a way that can mimic dark energy! Come find out how on this week’s Ask Ethan.
The Counterintuitive Reason Why Dark Energy Makes The Universe Accelerate
“In a nutshell, a new form of energy can affect the Universe’s expansion rate in a new way. It all depends on how the energy density changes over time. While matter and radiation get less dense as the Universe expands, space is still space, and still has the same energy density everywhere. The only thing that’s changed is our automatic assumption that we made: that energy ought to be zero. Well, the accelerating Universe tells us it isn’t zero. The big challenge facing astrophysicists now is to figure out why it has the value that it does. On that front, dark energy is still the biggest mystery in the Universe.”
There are lots of explanations out there for why the Universe’s expansion is accelerating. Some people point towards the negative pressure of a cosmological constant and talk about how this causes space to fly apart. Others call it a “fifth force” and imply that it’s a new fundamental relation that functions as some sort of anti-gravity. Neither of those explanations are correct, though, and they both complicate a much simpler (and more correct!) truth: that the Universe’s expansion rate is simply determined by all the different types of matter and energy within it. Dark energy is just another type of energy, but it’s different in a very particular way from the normal matter, dark matter, neutrinos, and radiation that we know.
Dark energy makes the Universe accelerate because of how it evolves and changes differently from everything else we know of over time. Come find out how!