Ask Ethan: How Large Is The Entire, Unobservable Universe?
“We know the size of the Observable Universe since we know the age of the Universe (at least since the phase change) and we know that light radiates. […] My question is, I guess, why doesn’t the math involved in making the CMB and other predictions, in effect, tell us the size of the Universe? We know how hot it was and how cool it is now. Does scale not affect these calculations?”
Our Universe today, to the best of our knowledge, has endured for 13.8 billion years since the Big Bang. But we can see farther than 13.8 billion light years, all because the Universe is expanding. Based the matter and energy present within it, we can determine that the observable Universe is 46.1 billion light years in radius from our perspective, a phenomenal accomplishment of modern science. But what about the unobservable part? What about the parts of the Universe that go beyond where we can see? Can we say anything sensible about how large that is?
We can, but only if we make certain assumptions. Come find out what we know (and think) past the limits of what we can see on this week’s Ask Ethan!
What Was It Like When The Universe Was At Its Hottest?
“At the inception of the hot Big Bang, the Universe reaches its hottest, densest state, and is filled with matter, antimatter, and radiation. The imperfections in the Universe — nearly perfectly uniform but with inhomogeneities of 1-part-in-30,000 — tell us how hot it could have gotten, and also provide the seeds from which the large-scale structure of the Universe will grow. Immediately, the Universe begins expanding and cooling, becoming less hot and less dense, and making it more difficult to create anything requiring a large or energy: E = mc2 means that creating a massive particle requires at least enough energy.
Over time, the expanding and cooling Universe will drive an enormous number of changes. But for one brief moment, everything was symmetric, and as energetic as possible. Somehow, over time, these initial conditions created the entire Universe.”
As soon as the Universe was filled with matter, antimatter, and radiation in the hot, dense state known as the Big Bang, it begins to expand and cool. For one brief moment, the Universe reached its maximum temperature and density, and had enough energy to spontaneously create anything at all that Einstein’s energy-mass equivalence would allow. But this state not only wouldn’t last, but it also was never arbitrarily or infinitely hot! There’s a limit to how energetic the Universe could have ever been, and we’ve determined it’s at least 1000 times smaller than the Planck scale. This is still trillions of times more energetic than anything the LHC ever created.
What was it like when the Universe was the hottest its ever been? Come find out on What-Was-It-Like-Whensday! (See what I did there?)
What Was It Like When The Big Bang First Began?
“Once inflation comes to an end, and all the energy that was inherent to space itself gets converted into particles, antiparticles, photons, etc., all the Universe can do is expand and cool. Everything smashes into one another, sometimes creating new particle/antiparticle pairs, sometimes annihilating pairs back into photons or other particles, but always dropping in energy as the Universe expands.
The Universe never reaches infinitely high temperatures or densities, but still attains energies that are perhaps a trillion times greater than anything the LHC can ever produce. The tiny seed overdensities and underdensities will eventually grow into the cosmic web of stars and galaxies that exist today. 13.8 billion years ago, the Universe as-we-know-it had its beginning. The rest is our cosmic history.”
The Big Bang is normally treated as the very beginning of the Universe, but in reality there’s a phase that came before the hot Big Bang to set it up. During cosmic inflation, the Universe was filled with an extremely large amount of energy inherent to space itself, causing the Universe to inflate, stretch flat, and achieve almost exactly the same properties everywhere. The Universe we have today, however, is full of matter and radiation, and originated in a hot Big Bang 13.8 billion years ago. How did we go from this inflating state to our hot, dense, uniform and expanding-and-cooling Universe?
This tells you the best scientific story of how we got there, along with an in-depth description of what it was like at those first moments where our Universe gave us something to look at.
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!
What Was It Like When The Universe Was Inflating?
“In theory, what lies beyond the observable Universe will forever remain unobservable to us, but there are very likely large regions of space that are still inflating even today. Once your Universe begins inflating, it’s very difficult to get it to stop everywhere. For every location where it comes to an end, there’s a new, equal-or-larger-sized location getting created as the inflating regions continue to grow. Even though most regions will see inflation end after just a tiny fraction of a second, there’s enough new space getting created that inflation should be eternal to the future.”
You’ve no doubt heard that the overwhelming scientific consensus is that the observable Universe began with the hot Big Bang. What’s far less common, but just as overwhelmingly accepted and well-understood, is that a period of cosmological inflation occurred prior to the Big Bang in order to set it up. While most of us can visualize the expanding Universe fairly well, it’s much more difficult to get a good handle on what the Universe looked like during the epoch of cosmic inflation. Yet if you want to know where our Universe came from, and how it was born with the properties our hot Big Bang started off with, that’s exactly the challenge you have to meet.
Here’s an in-depth but scientifically accurate description of what the Universe was like when inflation occurred, and how it gives us the Universe we inhabit today!
We Just Found The Missing Matter In The Universe, And Still Need Dark Matter
“For over 40 years, scientists have argued over dark matter’s existence.
Big questions arose from the motions inside galaxies, clusters of galaxies, and along the cosmic web.
From their gravity, we can infer the total mass in the Universe.
Yet multiple sources indicate that only 15% of that mass can be baryonic: made of normal matter.”
Is dark matter truly necessary? Many argued that, until we found the entirety of the normal matter in the Universe, we couldn’t be sure. The motions of galaxies, clusters of galaxies, and the formation of large-scale structure and the cosmic web all indicate a certain amount of mass in the Universe, and many sources such as the CMB and big bang nucleosynthesis indicate that the “normal” matter can only be about 15% of the total, implying dark matter. But finding all the normal matter has proven elusive, with the theorized WHIM (warm-hot intergalactic medium) not showing up in sufficient abundance. In particular, the hot part just wasn’t there.
Until now. Observation made with XMM-Newton have at last revealed it, and it’s there in just the right, predicted amounts. And therefore, dark matter is still absolutely necessary.
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.
Why Cosmology’s Expanding Universe Controversy Is An Even Bigger Problem Than You Realize
“The question of how quickly the Universe is expanding is one that has troubled astronomers and astrophysicists since we first realized that cosmic expansion was a necessity. While it’s incredibly impressive that two completely independent methods yield answers that are close to within less than 10%, the fact that they don’t agree with each other is troubling.
If the distance ladder group is in error, and the expansion rate is truly on the low end and near 67 km/s/Mpc, the Universe could fall into line. But if the cosmic microwave background group is mistaken, and the expansion rate is closer to 73 km/s/Mpc, we just may have a crisis in modern cosmology.
The Universe cannot have the dark matter density and initial fluctuations that such a value would imply. Until this puzzle is resolved, we must be open to the possibility that a cosmic revolution may be on the horizon.”
Ever since we first learned that the Universe was expanding, scientists have worked hard to measure just how fast that expansion rate is. From that, combined with what makes up the Universe, we can learn how old the Universe is and what it was like in the past, as well as what it’s fate will be in the future. Yet the two groups that make independent measurements of that rate, from the cosmic microwave background and the cosmic distance ladder, have gotten inconsistent results. If the distance ladder team has made a mistake, everything will be fine with cosmology. But if that team is right and the microwave background team is wrong, there should be a crisis coming.
Why is that? Come find out why the biggest controversy in modern cosmology might be an even bigger problem than almost everyone realizes!
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!
Ask Ethan: Will Future Civilizations Miss The Big Bang?
“If intelligent life re-emerges in our solar system in a few billion years, only a few points of light will still be visible in the sky. What kind of theory of the universe will those beings concoct? It is almost certain to be wrong. Why do we think that what we can view now can lead us to a “correct” theory when a few billion years before us, things might have looked completely different?”
Incredibly, the Universe we know and love today won’t be the way it is forever. If we were born in the far future, perhaps a hundred billion years from now, we wouldn’t have another galaxy to look at for a billion light years: hundreds of times more distant than the closest galaxies today. Our local group will merge into a single, giant elliptical galaxy, and there will be no sign at all of young stars, of star-forming regions, of other galaxies, or even of the Big Bang’s leftover glow. If we were born in the far future, we might miss the Big Bang as the correct origin of our Universe. It makes one wonder, when you think about it in those terms, if we’re missing something essential about our Universe today? In the 13.8 billion years that have passed, could we already have lost some essential information about the history of our Universe?
And in the far future, might we see something that, as of right now, hasn’t yet grown to prominence? Let’s explore this and see what you think!