Ask Ethan: Does Dark Energy Gravitate?
“Does dark energy gravititate? In other words does the increase in dark energy as space expands also create more gravity?”
Dark energy is the name we give to whatever’s responsible for the accelerated expansion of the Universe. According to our best theory of gravity, General Relativity, dark energy does indeed have an energy density, which doesn’t appear to be changing over time. This is bizarre, because for everything else, like matter and radiation, the fact that the Universe is expanding means that the density of “stuff” dilutes as time goes on. But for dark energy, it’s a form of energy that appears to be inherent to space itself, meaning that as the Universe expands, its density never goes down. You might think that adding more and more energy in the Universe would just cause it to gravitate more and more severely, though, eventually leading to a recollapse. That’s not what’s going on at all, though.
Why doesn’t dark energy lead to a recollapse? Does this mean the expanding Universe violates the conservation of energy? And what does it mean for dark energy to gravitate? Come get the answers today.
The Universe Would Be Very Different Without Dark Matter
“Finally, the smallest galaxies of all — the ones that contain only hundreds or thousands of stars — wouldn’t be able to exist at all. In our Universe, they arose from a clump of normal-and-dark matter of approximately 100,000 solar masses, where an episode of star formation ejected the gas. Still, the dark matter persisted, and holds the stars together in their own bound structure even to the present day. In a Universe without dark matter, that same episode of star formation would blast the proto-galaxy apart entirely, leaving only a slew of individual, unbound stars behind.
There are many different lines of evidence that point towards dark matter’s existence, but it’s perhaps a little more interesting to consider all the ways our Universe would be different — and inconsistent with what we observe — if it had no dark matter at all. If you enjoy the fact that the Universe is held together as well as it is, you have dark matter to thank for it. Even if you don’t believe in it, it’s a key ingredient in the Universe that led to you.”
Happy Valentine’s Day, dark matter! I know you don’t get nearly enough love on even a normal day of the year, so on this special day, don’t forget to carve out a piece of your heart for the unseen-but-important “glue” that holds the Universe together.
Without dark matter, the Universe as we know it, containing humans, life, and even something as basic as rocky planets, might not be possible at all.
How Far Is It To The Edge Of The Universe?
“If you define the edge of the Universe as the farthest object we could ever reach if we began our journey immediately, then our present limit is a mere distance of 18 billion light-years, encompassing just 6% of the volume of our observable Universe. If you define it as the limit of what we can observe a signal from — who we can see and who can see us — then the edge goes out to 46.1 billion light-years. But if you define it as the limits of the unobservable Universe, the only limit we have is that it’s at least 1,150 billion light-years in size, and it could be even larger.
This doesn’t necessarily mean that the Universe is infinite, though. It could be flat and still curve back on itself, with a donut-like shape known mathematically as a torus. As large and expansive as the observable Universe is, it’s still finite, with a finite amount of information to teach us. Beyond that, the ultimate cosmic truths still remain unknown to us.”
How far is it to the edge of the Universe? If you were to leave in a rocket ship today at the speed of light, what is the most distant object you’d be able to visit, and is that truly an edge? If you looked out at the most distant thing you could possibly observe, and the most distant location that could possibly observe us, how far would that be, and is that truly an edge? Or, would you consider the Universe beyond its observable limits, and wonder on the largest scales whose data exists only in our mind’s eye, whether there’s an edge at all?
Regardless of how you think about it, physics has answers, constraints, and limits for how far it truly is to the Universe’s edge. Find out how far it is today!
This Is How The Universe Changes With Every New Year That Passes
“With an 13.8 billion year lifetime so far, the Universe has certainly been around for some time. While it may seem to change only imperceptibly on human timescales, the fact remains that these changes are real, important, and cumulative. If we look closely and precisely enough, we can observe these changes on timescales as small as a single year.
These changes affect not only our home world, but our Solar System, galaxy, and even the entire Universe. We are only in the beginning stages of exploring how the Universe changes over time and what it looks like at the greatest distances and faintest extremes. May the 2020s mark the decade, at long last, where we pool our efforts as a species into the endeavor to uncover the greatest cosmic secrets of all.”
With every year that goes by, tiny, imperceptible changes occur in our physical Universe that really add up over time. The Earth’s rotation is slowing, the Moon is spiraling outwards, the Sun is heating up and new stars are forming. On a cosmic scale, the Universe is expanding and getting cooler, and more galaxies are becoming visible while fewer stars are capable of being visited.
This is how the Universe changes with each new year that passes, and we can quantify the effects today!
This Is Why December 31 Is The Cosmic Calendar’s Most Important Date
“All that human beings have ever accomplished occurs in a cosmic blink-of-an-eye. Our progress over the past few thousand years may have been rapid and incredible, and has brought us to a point where we now seek to extend our civilization beyond Earth. We have come so far in such a short time, cosmically speaking, but whether we’ll endure remains to be seen.
If we truly want to make any significant dent into “year 2” of the cosmic calendar, we have our work cut out for us. The world is changing rapidly and, on many fronts, we continue to damage and poison the ecosystem that sustains us. If we don’t begin taking a longer-term view of our civilization, we could be gone in mere cosmic seconds, just as all of recorded human history fits into mere seconds as well. As the 2010s give way to the 2020s, it’s up to all of us to pilot our one-and-only habitable planet, Earth, in the right direction.”
As the year comes to an end, it’s worth taking a look back at our cosmic history as though we had compressed our Universe’s entire 13.8 billion year existence into a single calendar year. If we did so, all of human history would only appear in the final few minutes of December 31, with a typical human lifetime lasting just 0.2 seconds. And yet, we can perceive the entire Universe, while trying our best to extend human civilization into the cosmic calendar’s “year 2.”
Here’s what we’re facing as we look back, and what we should be keeping in mind as we move forward. We’re all in this together.
Ask Ethan: Did God Create The Universe?
“I am very interested in space and with who made us and what made us… what do you have to say about people who say that “God” made us?”
In some ways, it’s the ultimate cosmic question. Before the Big Bang, before the final few moments of cosmic inflation, our knowledge ends. Not only does it end in the sense that we don’t know what came before that instant, but any information that ever existed is now definitively no longer present within our observable Universe. There is nothing we could ever look at, measure, observe, or experiment with, to the best of our knowledge and understanding, to reveal information about that epoch.
It might run contrary to science to appeal to the divine to explain the physical phenomena that have already been scientifically explained and understood, but it’s definitively true that science cannot and will not provide the answer to any and every question we can conceivably ask. For some questions, all science can do is remain agnostic, with every opinion on the matter remaining a matter of faith.
Regardless of what you believe, there’s a lesson for us all in how we treat one another, particularly if they believe differently from ourselves. It’s the last Ask Ethan of the 2010s, and if you’re only going to read one, this should be it!
What Is The Ultimate Fate Of The Loneliest Galaxy In The Universe?
“After an extraordinary amount of time has passed, googols of years or even more, the loneliest galaxy in the Universe will appear completely empty. No stars, stellar remnants, planetary corpses or even black holes ought to remain. And yet, it will still exist. Someone who could measure the spacetime curvature of the Universe or somehow detect dark matter or ultra-low energy neutrinos would encounter an enormous, diffuse halo of mass that will persist for far longer than any bound structure made of normal matter.
Eventually, dependent on the actual (and yet unknown) masses of individual dark matter particles and neutrinos, this remnant dark halo will decay, ejecting itself particle-by-particle until none remain. Until the masses and properties of those particles are known, however, we cannot calculate that timescale; we can only know it will persist longer than any normal matter will. The eventual fate of the last galaxies in the Universe will be a skeletal dark matter/neutrino halo, far outlasting anything else we’ve ever observed.”
Most of the galaxies we find in the Universe aren’t found in isolation, but exist bound to other galaxies, whether in a small group like our own or in an enormous grouping like the galaxies of the Virgo cluster. But out there, hundreds of millions of light-years away, galaxy MCG+01-02-015 exists in true isolation, with no other galaxies surrounding it for some 100 million light-years in all directions. Whereas we have tens of thousands of galaxies within that distance of ourselves, it has not even one. As a result, it’s a much cleaner astronomical laboratory, and we can predict its future far more certainly than we can predict our own.
So what is the ultimate fate of this galaxy: the loneliest one in the Universe? Come find out, with implications for everything else we know of, too!
This One Distant, Red, Gas-Free Galaxy Defies Astronomers’ Expectations
“When two similarly-sized galaxies merge, it triggers a starburst: a massive formation of new stars. Under the right circumstances, some gas will form stars while the remainder is expelled, lost forever to the intergalactic medium. Once the gas for forming new stars is used up, the galaxy simply ages as the bluest, most massive stars die off. Over billions of years, only the redder, dimmer, lower mass stars remain.”
In astronomy, young galaxies actively form stars, and glow bright blue through the process. Only after many billions of years and at least one cataclysmic event do galaxies settle down into a gas-free, red state, once all the bluer stars have died out. “Red and dead” galaxies appear in the late Universe, normally as giant elliptical galaxies that lost their gas aeons ago.
Which is why this one galaxy is so puzzling: it’s red, dead, massive and compact, but it’s also sending us its light from 10.8 billion years ago!
How did this galaxy get so old-looking when it’s actually so young? The mystery continues, but here’s what we know so far.
What Came First: Inflation Or The Big Bang?
“In fact, our entire observable Universe contains no signatures at all from almost all of its pre-hot-Big-Bang history; only the final 10^-32 seconds (or so) of inflation even leave observably imprinted signatures on our Universe. We do not know where the inflationary state came from, however. It might arise from a pre-existing state that does have a singularity, it might have existed in its inflationary form forever, or the Universe itself might even be cyclical in nature.
There are a lot of people who mean “the initial singularity” when they say “the Big Bang,” and to those people, I say it’s long past due for you to get with the times. The hot Big Bang cannot be extrapolated back to a singularity, but only to the end of an inflationary state that preceded it. We cannot state with any confidence, because there are no signatures of it even in principle, what preceded the very end-stages of inflation. Was there a singularity? Maybe, but even if so, it doesn’t have anything to do with the Big Bang.”
Have you heard that our Universe began some 13.8 billion years ago with the start of the Big Bang? There’s a good chance that some version of that story has made it to you, but it unfortunately has probably gotten to you the same way it got to me: with an error that’s many decades out of date.
What if I told you that you couldn’t extrapolate the Universe back to a singularity, where all the matter and energy was consolidated into a space so tiny that the laws of physics break down?
What if I told you that we have a verified, validated theory of what happened before the Big Bang, and it has (for decades, now) superseded and replaced the idea of an initial singularity as the earliest stages of the Universe?
Meet cosmic inflation, the pre-origin of our Universe that set up and gave rise to the Big Bang, and learn why the naysayers are out of legs to stand on.
Did Our Universe’s Structure Grow From The Top-Down Or From The Bottom-Up?
“A century ago, we didn’t even know what our Universe looked like. We didn’t know where it came from, whether or when it began, how old it was, what it was made out of, whether it was expanding, what was present within it. Today, we have scientific answers to all of these questions to within about 1% accuracy, plus a whole lot more.
The Universe was born almost perfectly uniform, with 1-part-in-30,000 imperfections present on practically all scales. The largest cosmic scales have slightly larger imperfections than the smaller ones, but the smaller ones are also substantial and collapse first. We likely formed the first stars just 50-to-200 million years after the Big Bang; the first galaxies arose 200-to-550 million years after the Big Bang; the largest galaxy clusters took billions of years to get there.
The Universe is neither top-down nor bottom-up, but a combination of both that implies it was born with an almost scale-invariant spectrum. With future survey telescopes such as LSST, WFIRST, and the next-generation of 30-meter-class ground-based telescopes, we’re poised to measure galaxy clustering as never before. After a lifetime of uncertainty, we can finally give a scientific answer to understanding how our Universe’s large-scale structure came to be.”
In a top-down scenario, the Universe would form structures on large scales first, then fragment to form individual galaxies. In a bottom-up scenario, the Universe forms tiny structures first, which then collect and clump under their own gravity to bring about a Universe rich in large-scale structure. So, which one is the Universe we have?
As is often the case, the answer is much more complex than just one of these two possibilities. Come get the full story today.