Posted on 10/28/2018 1:28:32 PM PDT by ETL
Let's talk about dark energy. We've known for about 20 years that the expansion of our universe is accelerating; every day, our cosmos grows bigger and bigger, doing so faster and faster. It's a subtle effect, and it takes extensive and deep cosmological surveys and studies for scientists to notice it. But multiple independent lines of evidence all point to the same conclusion: accelerating expansion.
Astronomers quickly cooked up a cool name for that accelerated expansion: dark energy. But now we’re left with the much harder job of finding a culprit — what's causing it?
We use general relativity, Albert Einstein's magnum opus, to understand gravity in all its manifestations, including the expansion of the universe. But the theory's equations have some wiggle room. Specifically, they allow for a so-called "cosmological constant," a fixed term that can be appended to the end. Adding this constant doesn't change the theory's descriptions of normal, everyday gravitational interactions, but it does make itself known when you're calculating the expansion of the universe.
Our natural inclination would be to set this constant to zero and forget about it, but Einstein himself introduced it because he found that without it, his relativity predicted a dynamic universe. At the time, both physicists and the general public thought of the cosmos as static and unchanging, so Einstein set a value for the constant to prevent those dynamic predictions. And then astronomer Edwin Hubble showed everyone that we do indeed live in an expanding universe, and Einstein realized that he'd missed a golden opportunity to predict that revolutionary observation. Oh, well.
But nowadays, we're faced with accelerated expansion, and the simplest explanation we have for it is that is that dark energy is simply Einstein's original cosmological constant. But the constant by itself is just a number — what's its physical significance?
In the 1960s, Soviet astrophysicist and all-around-genius Yakov Zel'dovich made a startling connection. The cosmological constant that appears in Einstein's equations is none other than the vacuum energy that quantum field theory predicts.
According to that theory, a suite of quantum fields permeates all of space-time. Sometimes, portions of these fields get excited and move around, and this is what we identify as particles. But left unperturbed, the fields are still associated with an energy. In other words, the empty vacuum of space-time has a raw energy, and that energy can be identified a the cosmological constant in general relativity, which means it might be the dark energy itself.
So, now that we have some sort of trail to follow, what do we predict the value of dark energy to be? The math isn't easy, but you just have to turn the quantum field theory crank, and out pops … infinity. Well, that's not going to work.
There are games you can play to make the predicted value for dark energy not infinity, but no matter what you do, you always end up with a very large number. But what about the actual, observed amount of dark energy, the one calculated from the accelerated expansion rate? It's very small: the energy equivalent of a little less than 1 hydrogen atom per cubic meter (35 cubic feet).
That "minor" discrepancy between dark energy's predicted value and the observed expansion rate is one of the biggest puzzles in modern physics. And its full resolution will probably come only with a true reckoning between quantum mechanics and general relativity.
Until then, we should probably understand how a vacuum energy can accelerate expansion.
This is one of those weird cases in which the math behind the physics is totally straightforward and unambiguous: Simply put, a constant vacuum energy causes the expansion of the universe to accelerate. But putting this conclusion into words so that we can intuitively understand it is another matter entirely, one that cosmologists have struggled with for decades.
I'm going to give it a shot myself, but if you just want to say to yourself, "because the math says so," and skip to the next section, I won't blame you.
Two key properties of a vacuum energy affect expansion. One is the vacuum's persistence; as the universe expands, there's more space, so there's more vacuum, so there's more vacuum energy. So, in our evolving cosmos, we find more and more dark energy lying around. The second vacuum property that's key for expansion is that the vacuum has tension (usually, confusingly, referred to as "negative pressure," but same deal). This tension resists the expansion of the universe; it's trying to rein in the expanding cosmos.
Put these two properties together and you get the complete opposite of what you may expect. This is because the equations of general relativity count all sources of energy to determine the behavior of the expansion of the universe, and different sources of energy can contribute positive or negative effects. So, the raw energy of the vacuum gets counted, which would be an attractive contribution, slowing down the expansion of the universe. But so does the vacuum's tension, which actually contributes repulsively., In other words, in the math of general relativity, the tension from dark energy carries a minus sign with it, and contributes to accelerating the expansion of the universe).
We use general relativity, Albert Einstein's magnum opus, to understand gravity in all its manifestations, including the expansion of the universe. But the theory's equations have some wiggle room. Specifically, they allow for a so-called "cosmological constant," a fixed term that can be appended to the end. Adding this constant doesn't change the theory's descriptions of normal, everyday gravitational interactions, but it does make itself known when you're calculating the expansion of the universe.
Our natural inclination would be to set this constant to zero and forget about it, but Einstein himself introduced it because he found that without it, his relativity predicted a dynamic universe. At the time, both physicists and the general public thought of the cosmos as static and unchanging, so Einstein set a value for the constant to prevent those dynamic predictions. And then astronomer Edwin Hubble showed everyone that we do indeed live in an expanding universe, and Einstein realized that he'd missed a golden opportunity to predict that revolutionary observation. Oh, well.
But nowadays, we're faced with accelerated expansion, and the simplest explanation we have for it is that is that dark energy is simply Einstein's original cosmological constant. But the constant by itself is just a number — what's its physical significance?
In the 1960s, Soviet astrophysicist and all-around-genius Yakov Zel'dovich made a startling connection. The cosmological constant that appears in Einstein's equations is none other than the vacuum energy that quantum field theory predicts.
According to that theory, a suite of quantum fields permeates all of space-time. Sometimes, portions of these fields get excited and move around, and this is what we identify as particles. But left unperturbed, the fields are still associated with an energy. In other words, the empty vacuum of space-time has a raw energy, and that energy can be identified a the cosmological constant in general relativity, which means it might be the dark energy itself.
So, now that we have some sort of trail to follow, what do we predict the value of dark energy to be? The math isn't easy, but you just have to turn the quantum field theory crank, and out pops … infinity. Well, that's not going to work.
There are games you can play to make the predicted value for dark energy not infinity, but no matter what you do, you always end up with a very large number. But what about the actual, observed amount of dark energy, the one calculated from the accelerated expansion rate? It's very small: the energy equivalent of a little less than 1 hydrogen atom per cubic meter (35 cubic feet).
That "minor" discrepancy between dark energy's predicted value and the observed expansion rate is one of the biggest puzzles in modern physics. And its full resolution will probably come only with a true reckoning between quantum mechanics and general relativity.
Until then, we should probably understand how a vacuum energy can accelerate expansion.
This is one of those weird cases in which the math behind the physics is totally straightforward and unambiguous: Simply put, a constant vacuum energy causes the expansion of the universe to accelerate. But putting this conclusion into words so that we can intuitively understand it is another matter entirely, one that cosmologists have struggled with for decades.
I'm going to give it a shot myself, but if you just want to say to yourself, "because the math says so," and skip to the next section, I won't blame you.
Two key properties of a vacuum energy affect expansion. One is the vacuum's persistence; as the universe expands, there's more space, so there's more vacuum, so there's more vacuum energy. So, in our evolving cosmos, we find more and more dark energy lying around. The second vacuum property that's key for expansion is that the vacuum has tension (usually, confusingly, referred to as "negative pressure," but same deal). This tension resists the expansion of the universe; it's trying to rein in the expanding cosmos.
Put these two properties together and you get the complete opposite of what you may expect. This is because the equations of general relativity count all sources of energy to determine the behavior of the expansion of the universe, and different sources of energy can contribute positive or negative effects. So, the raw energy of the vacuum gets counted, which would be an attractive contribution, slowing down the expansion of the universe. But so does the vacuum's tension, which actually contributes repulsively., In other words, in the math of general relativity, the tension from dark energy carries a minus sign with it, and contributes to accelerating the expansion of the universe).
It turns out that general relativity cares more about pressure and tension than raw energy. This is something wholly unfamiliar to us in our everyday experience; in almost all other cases, such as the motions in the solar system or even in the vicinity of black holes, the pressure/tension doesn't matter in the calculation. But it matters here.
The ultimate irony is that even though we're getting more and more dark energy every day, which would normally try to shrink the cosmos, its own resistance to that expansion causes the whole equation to flip around and instead accelerate the universe's growth.
The above explanation may or may not have been satisfying. Again, the math is clear, but there is no simple way to translate those equations to English. Nevertheless, the behavior of vacuum energy is currently our best explanation for the accelerating expansion of the universe. And it's not the greatest one. As we saw, we are severely lacking in our ability to predict the amount of acceleration.
While the existence of dark energy is beyond doubt, its cause is ultimately unknown. We have some likely suspects but no hard evidence. We need more sleuthing, measuring the universe's expansion rate and the history of the expansion rate to ever-greater precision. But because dark energy is a very subtle effect, it will take an entirely new generation of observations to (hopefully) reveal some answers.
_____________________________________________________________
Paul Sutter is an astrophysicist at The Ohio State University and the chief scientist at COSI science center. Sutter is also host of "Ask a Spaceman" and "Space Radio" and leads AstroTours around the world. Sutter contributed this article to Space.com's Expert Voices: Op-Ed & Insights.
I don’t get it. Maybe if I watched “Ant-Man and the Wasp,” where they introduce the Quantum Realm, it would make more sense.
It’s likely the QR will be in play in Avengers 4.
Sorry, screwed up a bit on the excerpt, duplicating two sections of it.
“Sorry, screwed up a bit on the excerpt, duplicating two sections of it.’
True, but you more than made up for it with imaginative art renderings.
Here are some good documentaries/lectures on the general subject...
Dark Energy: The Biggest Mystery in the Universe
https://www.youtube.com/watch?v=SvIhi8PZJhk
2018 Buhl Lecture: Exploding Stars, Dark Energy and the Accelerating Universe by Robert Kirshner
https://www.youtube.com/watch?v=Gg7j3g7cWFM
Inflationary cosmology on trial
https://www.youtube.com/watch?v=IcxptIJS7kQ
That 2nd one is for real...
The Hubble Deep Field (HDF) is an image of a small region in the constellation Ursa Major, constructed from a series of observations by the Hubble Space Telescope.
It covers an area about 2.6 arcminutes on a side, about one 24-millionth of the whole sky, which is equivalent in angular size to a tennis ball at a distance of 100 metres.[1]
The image was assembled from 342 separate exposures taken with the Space Telescope’s Wide Field and Planetary Camera 2 over ten consecutive days between December 18 and December 28, 1995.[2][3]
The field is so small that only a few foreground stars in the Milky Way lie within it; thus, almost all of the 3,000 objects in the image are galaxies, some of which are among the youngest and most distant known.
By revealing such large numbers of very young galaxies, the HDF has become a landmark image in the study of the early universe.
Three years after the HDF observations were taken, a region in the south celestial hemisphere was imaged in a similar way and named the Hubble Deep Field South.
The similarities between the two regions strengthened the belief that the universe is uniform over large scales and that the Earth occupies a typical region in the Universe (the cosmological principle).
A wider but shallower survey was also made as part of the Great Observatories Origins Deep Survey. In 2004 a deeper image, known as the Hubble Ultra-Deep Field (HUDF), was constructed from a few months of light exposure.
The HUDF image was at the time the most sensitive astronomical image ever made at visible wavelengths, and it remained so until the Hubble eXtreme Deep Field (XDF) was released in 2012.
https://en.wikipedia.org/wiki/Hubble_Deep_Field
Hubble’s observations about the expanding universe contributed to discoveries showing that time, space, matter and energy all had a beginning.
This contradicted the ultimately irrational cosmology Einstein had accepted along with most scientists of his day—the steady state universe existing infinitely into the past and future.
Bookmark to read later.
“almost all of the 3,000 objects in the image are galaxies”
That’s truly astounding. I get algebra, trig and have more than a rudimentary understanding of calculus, but cosmology and quantum theory are beyond my ken (even though I passed a course or two on quantum theory).
Apparently, the takeaway from this is dark matter SUCKS!
Fill a horsetank with water to the very top, then cover it with a thin rubber sheet and seal it so no air is in it and no water can escape.
Then drop a bowling ball in the center...
The ball will make a dent but furthest away from the ball the surface will be BULGED UPWARDS!
If the sealed tank with no ball is no gravity then the tanke with bowling ball with the outward bulge upwards is anti-gravity... ie. NEGATIVE ENERGY ie. REPULSIVE FORCE.
DUH!
The classic thought experiment of the “trampoline + bowling ball “ gravity model crap missed the idea of the displacement in the 3rd dimension...of a non compressible lower/higher dimensional space time metric....
Is the horse spherical?
[ Is the horse spherical? ]
No but the tank is cylindrical.
4Ltr
Expanding into what? That just sounds silly. Is the cosmos minuscule compared to the emptiness it has not yet expanded into?
Thanks for posting, ETL.
Grace G, thanks for posting your horse tank analogy, which I have not seen or thought about before.
“almost all of the 3,000 objects in the image are galaxies”
That’s truly astounding.
...
What I find astounding is that at one time all those galaxies and many more came from a tiny dot smaller than an atom.
Another astounding thought is that all the matter we see is but a wisp of smoke in the larger Universe.
But if you have a Quantum view of Physics this is all wrong.
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