How big is space? Interactive views of the universe in varying scales
December 4, 2014 1:35 PM Subscribe
We know space is big, but trying to understand how big is tricky. Say you stare up at the sky and identify stars and constellations in a virtual planetarium, you can't quite fathom how far away all those stars are (previously, twice). Even if you could change your point of view and zoom around in space to really see 100,000 nearby stars (autoplaying ambient music, and there are actually 119,617 stars mapped in 3D space), it's still difficult to get a sense of scale. There's this static image of various items mapped on a log scale from XKCD (previously), and an interactive horizontal journey down from the sun to the heliosphere with OMG Space (previously). You can get a bit more dynamic with this interactive Scale of the Universe webpage (also available in with some variants, if you want the sequel [ previously, twice], the swirly, gravity-optional version that takes some time to load, and the wrong version [previously]), but that's just for the scale of objects, not of space itself. If you want to get spaced out, imagine if If the Moon Were Only 1 Pixel, and travel from there (previously). This past March, BBC Future put out a really big infographic, which also takes a moment to load, but then you can see all sorts of things, from the surface of Earth out to the edge of our solar system.
Also: if you want to feel real small, download Space Engine and fly around the universe.
posted by hellojed at 1:59 PM on December 4, 2014 [1 favorite]
posted by hellojed at 1:59 PM on December 4, 2014 [1 favorite]
Wow voyager I traveling at 58620 km per hour. How'd it get so fast. It's weird to think about Voyager I just out there.
posted by rudster at 2:09 PM on December 4, 2014
posted by rudster at 2:09 PM on December 4, 2014
It's interesting to note that we have literally no idea how big the universe is. The observable universe is at least 93 billion light years across, but it is certainly much larger and quite possibly infinite.
One thing that I find mind-boggling about that fact is that since the upper bound is infinity, and we have no other universes as data points, we can't even assign probabilities to any ranges of size. Like, we can't say there's a 90% chance that it's smaller than X light years, for any finite value of X, or that it's larger than Y for any value of Y over the minimum. That melts my brain.
posted by justkevin at 2:12 PM on December 4, 2014 [3 favorites]
One thing that I find mind-boggling about that fact is that since the upper bound is infinity, and we have no other universes as data points, we can't even assign probabilities to any ranges of size. Like, we can't say there's a 90% chance that it's smaller than X light years, for any finite value of X, or that it's larger than Y for any value of Y over the minimum. That melts my brain.
posted by justkevin at 2:12 PM on December 4, 2014 [3 favorites]
When the topic of a huge universe comes up, I like to point at my favorite personal project. I last updated it about 8 years ago, but I still love to revisit it from time to time:
MMBU Cow Universe Video
I really should go back and do more with that site. Maybe update the look to be more modern, add some more comparisons, perhaps add Android intregration so people can compare themselves to the universe.
posted by HappyEngineer at 2:39 PM on December 4, 2014
MMBU Cow Universe Video
I really should go back and do more with that site. Maybe update the look to be more modern, add some more comparisons, perhaps add Android intregration so people can compare themselves to the universe.
posted by HappyEngineer at 2:39 PM on December 4, 2014
Please correct me if I've been making this observation incorrectly but looking at a globe, the paint on the surface is proportionally thicker than the atmosphere.
posted by sammyo at 3:05 PM on December 4, 2014
posted by sammyo at 3:05 PM on December 4, 2014
Exoplant is one of my absolutely favorite iPad apps, it blows my mind to think that such an amazing thing can be free, but for a dollar IAP, you can grab the database of Messier objects (including galaxies), and zoom around the Universe. It also displays the dense asteroid belt in our solar system, and it shows how far the Voyager and Pioneer probes are from the Earth (not as far as you'd think, cosmology-wise). With this slick little program, you can really visualize scale in a way that no text can convey.
posted by dbiedny at 3:08 PM on December 4, 2014 [4 favorites]
posted by dbiedny at 3:08 PM on December 4, 2014 [4 favorites]
Correct me if I'm wrong, which I probably am, but the size of the universe is dependent on the velocity of the observer, right? If I were traveling at the speed of light, the universe would have no size because wherever I "went" I would leave and arrive at exactly the same time which means no distance could have been covered. Or, to put it another way, in what sense is it meaningful to speak of the size of the entire universe?
posted by haricotvert at 4:51 PM on December 4, 2014
posted by haricotvert at 4:51 PM on December 4, 2014
haricotvert: "If I were traveling at the speed of light, the universe would have no size because wherever I "went" I would leave and arrive at exactly the same time which means no distance could have been covered."
Nope.
posted by signal at 5:06 PM on December 4, 2014
Nope.
posted by signal at 5:06 PM on December 4, 2014
'Space. It seems to go on and on forever. Then you get to the end, and a monkey starts throwing barrels at you.'
Phillip Fry
posted by Hello, I'm David McGahan at 5:21 PM on December 4, 2014 [1 favorite]
Phillip Fry
posted by Hello, I'm David McGahan at 5:21 PM on December 4, 2014 [1 favorite]
I did ask to be corrected if I'm wrong. "Nope" would be somewhat short of the explanation I was seeking. Or perhaps it's a full explanation of my error accelerated to near the speed of light?
posted by haricotvert at 5:22 PM on December 4, 2014
posted by haricotvert at 5:22 PM on December 4, 2014
sammyo , not far off. I have a nice 16-inch globe (= 40 cm, give or take) which seems to be papered; I find two reams of paper, 1000 sheets, to be about 10 cm thick. So my globe has 200 mm radius and 0.1 mm surface. Compare to the earth which has 6400 km radius and, for rounding purposes, 7 km of troposphere (the "normal" part of the atmosphere). So the paper on the globe is thinner than the to-scale troposphere by a factor of at least two. If you have a smaller desk-sized globe, you might have the decoration layer thicker than the model troposphere. But that's using a smallish height estimate for the troposphere, and forgetting completely about the stratosphere (ten times taller) and mesosphere.
posted by fantabulous timewaster at 5:23 PM on December 4, 2014 [2 favorites]
posted by fantabulous timewaster at 5:23 PM on December 4, 2014 [2 favorites]
haricotvert: Depends on what you mean by the size of the universe. Usually what people mean is the distance traveled by photons in the cosmic microwave background, which was emitted about 13.8 billion years ago (Gya) when the universe went from being a hot ionized plasma, like the surface of the sun, to an electrically neutral gas. So it's common to think of the universe as being 13.8 billion light-years (Gly) in radius. But the expansion of the universe has carried those objects away from us since then; they're now about 45 Gly away from us, and being carried away by cosmic expansion faster than the speed of light. (I'll just admit that I have a little trouble keeping the proper terminology straight.) And furthermore because we can see that the microwave background is very uniform in temperature, we have to infer that any edge or boundary or significant change in the structure of the universe is much larger than the portion inside of our observable horizon; trillions of light-years, most likely. That's just the radius. The volume of the universe that we can see is a very tiny fraction of the universe's size.
In fact everything farther than 10 Gly or 15 Gly is beyond our "communication horizon": because the expansion of the universe is accelerating, any signal which we send towards them (such as a Metafilter commenter traveling near the speed of light) will never actually arrive. To the extent that it makes sense to talk about a speed-of-light observer finding the location of the CMB horizon, it actually wouldn't be terribly deformed from the sphere that we see.
There are lots of other issues at play, too. Cosmology is complicated.
posted by fantabulous timewaster at 5:50 PM on December 4, 2014 [6 favorites]
In fact everything farther than 10 Gly or 15 Gly is beyond our "communication horizon": because the expansion of the universe is accelerating, any signal which we send towards them (such as a Metafilter commenter traveling near the speed of light) will never actually arrive. To the extent that it makes sense to talk about a speed-of-light observer finding the location of the CMB horizon, it actually wouldn't be terribly deformed from the sphere that we see.
There are lots of other issues at play, too. Cosmology is complicated.
posted by fantabulous timewaster at 5:50 PM on December 4, 2014 [6 favorites]
If I were traveling at the speed of light, the universe would have no size because wherever I "went" I would leave and arrive at exactly the same time which means no distance could have been covered.
Light takes time to travel - it isn't instantaneous. You've probably heard the term light-year, which is a unit of length used informally to express astronomical distances. It is equal to just under 10 trillion kilometres (or about 6 trillion miles). As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year.
posted by filthy light thief at 5:52 PM on December 4, 2014
Light takes time to travel - it isn't instantaneous. You've probably heard the term light-year, which is a unit of length used informally to express astronomical distances. It is equal to just under 10 trillion kilometres (or about 6 trillion miles). As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year.
posted by filthy light thief at 5:52 PM on December 4, 2014
We speak of the size of the Universe in the space-time foliation in which the CMB is isotropic.
In words that are not designed to convince you I went to grad school, when we talk about the size of the visible Universe, we are talking about the size in the rest-frame in which the background radiation from the afterglow of the Big Bang (the Cosmic Microwave Background - CMB) doesn't have a direction that appears to be hotter because of an observer's relative motion towards it.
Functionally, this nearly the same as the Earth's rest frame. The Sun is moving relative to the CMB at 371 km/s, which is pretty damn fast, but only 0.001c, so the relative gamma factor (the time-dilation or length-contraction factor) is about 1.000001. So we have to do corrections for this, but it's not a big change.
We use this rest frame because this is effectively the rest frame of the Big Bang. I do lose sleep occasionally wondering how the Universe picked that frame, but it had to pick something, so probably no big mystery there. So it picked that particular frame, and everything in the Universe that wasn't accelerated to huge velocities somehow is moving relatively slowly relative to that frame.
In this frame, working out the evolution of the Universe over time (to calculate, for example, the age or size) is much more straightforward than any other frame, which is why we use it. That should make sense: this is the frame that the Universe is flat and isotropic, so there's no special direction, and things are approximately the same everywhere which makes evolving differential equations forward in time tractable. The flatness and isotropic nature of the Universe is a non-trivial fact, you could have built a Universe much like ours that was not. Given that our Universe happens to have these properties (see my posts here on inflation for why that might be), we can speak of the size and age of the Universe in the unique special frame that the Universe itself picked.
Someone moving at 99.999999999.......% of the speed of light since the birth of the Universe would measure different lengths and ages relative to themselves, but they would also see a CMB massively blueshifted in one direction and redshifted in the other, and so they too could work out that there is a special reference frame and all these Universal properties in that frame, same as any other observer (like us). Then they'd better start wondering what the hell they did to get moving so fast relative to it.
For a photon, of course, time does not exist, and the age of the Universe is a meaningless statement. Of course, photons also don't have much of a personality, so asking them about the timelessness of time is not a useful exercise.
posted by physicsmatt at 5:59 PM on December 4, 2014 [8 favorites]
In words that are not designed to convince you I went to grad school, when we talk about the size of the visible Universe, we are talking about the size in the rest-frame in which the background radiation from the afterglow of the Big Bang (the Cosmic Microwave Background - CMB) doesn't have a direction that appears to be hotter because of an observer's relative motion towards it.
Functionally, this nearly the same as the Earth's rest frame. The Sun is moving relative to the CMB at 371 km/s, which is pretty damn fast, but only 0.001c, so the relative gamma factor (the time-dilation or length-contraction factor) is about 1.000001. So we have to do corrections for this, but it's not a big change.
We use this rest frame because this is effectively the rest frame of the Big Bang. I do lose sleep occasionally wondering how the Universe picked that frame, but it had to pick something, so probably no big mystery there. So it picked that particular frame, and everything in the Universe that wasn't accelerated to huge velocities somehow is moving relatively slowly relative to that frame.
In this frame, working out the evolution of the Universe over time (to calculate, for example, the age or size) is much more straightforward than any other frame, which is why we use it. That should make sense: this is the frame that the Universe is flat and isotropic, so there's no special direction, and things are approximately the same everywhere which makes evolving differential equations forward in time tractable. The flatness and isotropic nature of the Universe is a non-trivial fact, you could have built a Universe much like ours that was not. Given that our Universe happens to have these properties (see my posts here on inflation for why that might be), we can speak of the size and age of the Universe in the unique special frame that the Universe itself picked.
Someone moving at 99.999999999.......% of the speed of light since the birth of the Universe would measure different lengths and ages relative to themselves, but they would also see a CMB massively blueshifted in one direction and redshifted in the other, and so they too could work out that there is a special reference frame and all these Universal properties in that frame, same as any other observer (like us). Then they'd better start wondering what the hell they did to get moving so fast relative to it.
For a photon, of course, time does not exist, and the age of the Universe is a meaningless statement. Of course, photons also don't have much of a personality, so asking them about the timelessness of time is not a useful exercise.
posted by physicsmatt at 5:59 PM on December 4, 2014 [8 favorites]
Thanks for those terrific explanations, physicsmatt and fantabulous time waster. Very much appreciated!
Do astronomers believe that the universe actually exists past the point where its expansion exceeds light speed? It sounds to me like there would be some kind of "event horizon" due to the cosmic expansion -- that objects would appear to us to move slower and slower due to their acceleration away until they hit light speed and just stopped. Is that true, and if so, wouldn't that really be the end of the universe for all practical purposes?
posted by haricotvert at 6:50 PM on December 4, 2014
Do astronomers believe that the universe actually exists past the point where its expansion exceeds light speed? It sounds to me like there would be some kind of "event horizon" due to the cosmic expansion -- that objects would appear to us to move slower and slower due to their acceleration away until they hit light speed and just stopped. Is that true, and if so, wouldn't that really be the end of the universe for all practical purposes?
posted by haricotvert at 6:50 PM on December 4, 2014
Not that anyone's still following this thread, but I researched my own question and it appears the answer is complicated in ways you'd have to know a lot of physics to understand. My poor understanding is that maybe there is an event horizon to the universe for each observer but it depends on the model you use and it's also possible that if the rate of inflation decreases enough then information emitted by a body that temporarily exceeded the speed of light could again become observable. But it sounds very technical, and makes me feel like my question -- where "was" the body when it was outside the causal sphere for that observer -- is probably just a misunderstanding.
posted by haricotvert at 5:29 AM on December 5, 2014 [1 favorite]
posted by haricotvert at 5:29 AM on December 5, 2014 [1 favorite]
Using the size of the universe as a unit of measure, the universe is exactly 1 wide. I don't see why that's so difficult.
posted by blue_beetle at 5:44 AM on December 5, 2014 [1 favorite]
posted by blue_beetle at 5:44 AM on December 5, 2014 [1 favorite]
Your question is very good, and the answer is complicated because not only are there multiple ideas that need to be conveyed, but there are different answers depending on what sort of Universe we happen to be living in.
Executive summary: in our Universe, there exists a cosmic event horizon, beyond which we can never see the light from events occurring "now," due to the expansion of the Universe.
Now let's get into it. When we do cosmology, we work in a narcissistic frame of reference where we, the people doing the calculation, live in the center of the coordinate frame, at rest. Everything else then is receding away from us. Now, we could, if we wanted, translate our results over to another point in space, in which case that point would be at rest and everything recedes away from them, including us at the original center point.
The usual analogy is living on the surface of an inflating balloon: everyone on the balloon sees themselves at rest and the remainder of the surface moving away from them. The true "center" of the balloon is in a coordinate orthogonal to the surface, just as the "center" of the Universe is orthogonal to our 3-D space (it's back in time). However, any observer on the balloon surface could call themselves the "center" of the 2-D surface and work from there, and so any observer in the Universe can call their location the center of the 3-D surface for mathematical simplicity. (This assumes they are in the CMB rest frame, otherwise the observer sees special directions, but we already went over that.)
OK, so we sit in the center of our coordinate frame, and look around us. Since there is a cosmic speed limit, information can only propagate at the speed of light (or slower), so we are not aware of what's going on "now" elsewhere in the Universe. (Again, everything I'm going to say here will be using a specific slicing of time relative to the rest frame of the CMB) We have to wait until the light reaches us. Of course, as the light moves towards us, the Universe stretches due to cosmic expansion, and so the light takes longer to reach us than you would have expected if you just took the initial distance between the origin of the light and us at time of emission and divided that length by the speed of light. The light will also be red-shifted as it travels.
This means that at any given moment, there is the particle horizon: the furthest distance away from us at any moment at which we can see events occurring. Obviously, as the Universe gets older, our particle horizon gets larger: we can see "more" of the Universe because we have more time for the light to reach us. The size of the particle horizon at any moment depends on how the Universe's size changed at every moment from the "start" to today. That is, it is related to the integral of the rate of expansion of the Universe. This is driven by the types of energy density in the Universe, which we measure by a combination of particle physics (calculating how radiation drives expansion compared to non-relativistic massive particles, for example) and direct measurements of how fast distant objects seem to be receding from us as a function of their apparent distance.
Now, if the Universe was built only of matter and radiation, the particle horizon grows to infinity. That is, if you wait an infinite amount of time, you can, in principle, see infinitely distant events. Matter and radiation have specific meanings here: matter density dilutes as cube of the length scale increase, because if you have a room with matter in it, and make each side of the room twice as long while keeping the total amount of stuff the same, you have 1/2^3 the density. Radiation goes like scale^4, since the energy of radiation is in the wavelength, and that redshifts as the Universe expands.
There's a way of picturing this, called a Penrose diagram. I started trying to explain those here, but it's really hard to do without a picture, and so maybe I'll have to take it off-site.
Anyway, the point is that in a Universe of only matter and radiation, you can if you are patient see whatever you want, just due to how the Universe expands. However, we don't live in such a Universe. Our Universe has dark energy. Now, we don't know what the hell this stuff "is" on a level that makes a particle physicist like me happy. We don't know even how exactly it dilutes as the Universe expands. It is consistent, however, with not diluting at all as you increase the size of the Universe. Which is pretty nuts if you think about it. But such things are possible. If dark energy has that precise property, it's a "cosmological constant" (it could also dilute a little as the Universe expands, or even increase in density a little as the Universe expands. We need to make more precise measurements of the expansion history to reduce the error bars.)
With a cosmological constant, the expansion rate of the Universe will keep increasing. Since a cosmological constant is constant, eventually it will be the only stuff around that is important in terms of how the Universe expands, regardless of how much matter or radiation you started with, or how small your cosmological constant value is. Today, the Universe is 68% dark energy, but earlier in time it was less dominated by this type of energy. In the future, the relative amount of dark energy will grow asymptotically to 100%. The energy density of dark energy (Lambda) remains constant though (again, assuming it's a perfect cosmological constant).
In such a Universe, there is an event horizon: a distance beyond which the light from events that occur will never reach you, no matter how long you wait. This horizon is small if the cosmological constant is big, and large if the constant is small (small Lambda means small rate of acceleration, so we should see further before the Universe's acceleration kicks in enough). So our event horizon goes like 1/Lambda.
So, in a Universe like the one we thought we lived in prior to 1998, you can see everything, assuming you're immortal, patient, and willing to build impossibly good telescopes that can see impossibly low energies. In the real Universe, the one with a small non-zero Lambda, that one you can't. We have a horizon, and beyond that we can have no idea of what's going on. Ever.
Now, just to bend brains a bit more before I leave, I will remind the observant reader of a few things. We're talking about an event horizon, which defines a boundary between what I can in principle see and communicate with and what I can't. Black holes also have event horizons. Hawking proved that black hole event horizons radiate. They have to radiate because black holes have entropy, and they have to have entropy because otherwise there's a way to violate the 2nd law of thermodynamics (throw a high entropy system into a black hole. If the black hole doesn't have entropy, viola, you have the perfect trash compactor for entropy. This would be bad for physics. ) Later, it was realized that black hole entropy means that all the information inside a volume can be painted somehow onto the 2D surface of the horizon around the volume. This makes very little sense from the perspective of quantum field theory, but appears to be true. Look up my posts here about the Holographic Principle.
Since our Universe has Lambda not equal to zero (such a Universe is called a 'de Sitter' universe), we too have a horizon. Every observer has an event horizon, but that horizon is different for each observer, just as every observer can see themselves as the center point from which everything expands away from. All these horizons also have entropy, and radiate. According the Holographic Principle, everything that is occurring or can occur or will occur in our visible Universe is somehow painted onto the 2D cosmological event horizon. Fun, isn't it?
posted by physicsmatt at 6:50 AM on December 5, 2014 [17 favorites]
Executive summary: in our Universe, there exists a cosmic event horizon, beyond which we can never see the light from events occurring "now," due to the expansion of the Universe.
Now let's get into it. When we do cosmology, we work in a narcissistic frame of reference where we, the people doing the calculation, live in the center of the coordinate frame, at rest. Everything else then is receding away from us. Now, we could, if we wanted, translate our results over to another point in space, in which case that point would be at rest and everything recedes away from them, including us at the original center point.
The usual analogy is living on the surface of an inflating balloon: everyone on the balloon sees themselves at rest and the remainder of the surface moving away from them. The true "center" of the balloon is in a coordinate orthogonal to the surface, just as the "center" of the Universe is orthogonal to our 3-D space (it's back in time). However, any observer on the balloon surface could call themselves the "center" of the 2-D surface and work from there, and so any observer in the Universe can call their location the center of the 3-D surface for mathematical simplicity. (This assumes they are in the CMB rest frame, otherwise the observer sees special directions, but we already went over that.)
OK, so we sit in the center of our coordinate frame, and look around us. Since there is a cosmic speed limit, information can only propagate at the speed of light (or slower), so we are not aware of what's going on "now" elsewhere in the Universe. (Again, everything I'm going to say here will be using a specific slicing of time relative to the rest frame of the CMB) We have to wait until the light reaches us. Of course, as the light moves towards us, the Universe stretches due to cosmic expansion, and so the light takes longer to reach us than you would have expected if you just took the initial distance between the origin of the light and us at time of emission and divided that length by the speed of light. The light will also be red-shifted as it travels.
This means that at any given moment, there is the particle horizon: the furthest distance away from us at any moment at which we can see events occurring. Obviously, as the Universe gets older, our particle horizon gets larger: we can see "more" of the Universe because we have more time for the light to reach us. The size of the particle horizon at any moment depends on how the Universe's size changed at every moment from the "start" to today. That is, it is related to the integral of the rate of expansion of the Universe. This is driven by the types of energy density in the Universe, which we measure by a combination of particle physics (calculating how radiation drives expansion compared to non-relativistic massive particles, for example) and direct measurements of how fast distant objects seem to be receding from us as a function of their apparent distance.
Now, if the Universe was built only of matter and radiation, the particle horizon grows to infinity. That is, if you wait an infinite amount of time, you can, in principle, see infinitely distant events. Matter and radiation have specific meanings here: matter density dilutes as cube of the length scale increase, because if you have a room with matter in it, and make each side of the room twice as long while keeping the total amount of stuff the same, you have 1/2^3 the density. Radiation goes like scale^4, since the energy of radiation is in the wavelength, and that redshifts as the Universe expands.
There's a way of picturing this, called a Penrose diagram. I started trying to explain those here, but it's really hard to do without a picture, and so maybe I'll have to take it off-site.
Anyway, the point is that in a Universe of only matter and radiation, you can if you are patient see whatever you want, just due to how the Universe expands. However, we don't live in such a Universe. Our Universe has dark energy. Now, we don't know what the hell this stuff "is" on a level that makes a particle physicist like me happy. We don't know even how exactly it dilutes as the Universe expands. It is consistent, however, with not diluting at all as you increase the size of the Universe. Which is pretty nuts if you think about it. But such things are possible. If dark energy has that precise property, it's a "cosmological constant" (it could also dilute a little as the Universe expands, or even increase in density a little as the Universe expands. We need to make more precise measurements of the expansion history to reduce the error bars.)
With a cosmological constant, the expansion rate of the Universe will keep increasing. Since a cosmological constant is constant, eventually it will be the only stuff around that is important in terms of how the Universe expands, regardless of how much matter or radiation you started with, or how small your cosmological constant value is. Today, the Universe is 68% dark energy, but earlier in time it was less dominated by this type of energy. In the future, the relative amount of dark energy will grow asymptotically to 100%. The energy density of dark energy (Lambda) remains constant though (again, assuming it's a perfect cosmological constant).
In such a Universe, there is an event horizon: a distance beyond which the light from events that occur will never reach you, no matter how long you wait. This horizon is small if the cosmological constant is big, and large if the constant is small (small Lambda means small rate of acceleration, so we should see further before the Universe's acceleration kicks in enough). So our event horizon goes like 1/Lambda.
So, in a Universe like the one we thought we lived in prior to 1998, you can see everything, assuming you're immortal, patient, and willing to build impossibly good telescopes that can see impossibly low energies. In the real Universe, the one with a small non-zero Lambda, that one you can't. We have a horizon, and beyond that we can have no idea of what's going on. Ever.
Now, just to bend brains a bit more before I leave, I will remind the observant reader of a few things. We're talking about an event horizon, which defines a boundary between what I can in principle see and communicate with and what I can't. Black holes also have event horizons. Hawking proved that black hole event horizons radiate. They have to radiate because black holes have entropy, and they have to have entropy because otherwise there's a way to violate the 2nd law of thermodynamics (throw a high entropy system into a black hole. If the black hole doesn't have entropy, viola, you have the perfect trash compactor for entropy. This would be bad for physics. ) Later, it was realized that black hole entropy means that all the information inside a volume can be painted somehow onto the 2D surface of the horizon around the volume. This makes very little sense from the perspective of quantum field theory, but appears to be true. Look up my posts here about the Holographic Principle.
Since our Universe has Lambda not equal to zero (such a Universe is called a 'de Sitter' universe), we too have a horizon. Every observer has an event horizon, but that horizon is different for each observer, just as every observer can see themselves as the center point from which everything expands away from. All these horizons also have entropy, and radiate. According the Holographic Principle, everything that is occurring or can occur or will occur in our visible Universe is somehow painted onto the 2D cosmological event horizon. Fun, isn't it?
posted by physicsmatt at 6:50 AM on December 5, 2014 [17 favorites]
Yes, since crossing the visible universe requires a time comparable to the age of the universe, you can't do it without invoking some thorny cosmology questions.
posted by fantabulous timewaster at 7:05 AM on December 5, 2014
posted by fantabulous timewaster at 7:05 AM on December 5, 2014
Holy s**t. That was a sensational explanation, physicsmatt. Do you have a tip jar?
posted by haricotvert at 8:38 AM on December 5, 2014
posted by haricotvert at 8:38 AM on December 5, 2014
I suppose one point that might help is to clarify when we say "expansion of the universe" we aren't meaning "movement of matter away from the "center" or origin point of the big bang" but that space itself is literally expanding, not just a uniform cosmic empty space where "the universe" (i.e. the matter/energy we think of as "stuff") expands into. That's one of the trickiest ideas to get your head around, though once you grok that it helps simplify a lot of stuff.
It sounds that haricotvert groks that, but figured I'd drop that in for people who are reading this who might not.
I mean I think my understanding is (mostly) right in that regards?
posted by symbioid at 8:44 AM on December 5, 2014
It sounds that haricotvert groks that, but figured I'd drop that in for people who are reading this who might not.
I mean I think my understanding is (mostly) right in that regards?
posted by symbioid at 8:44 AM on December 5, 2014
Right. The most common comparison is to a loaf of raisin bread being baked and expanding (in all directions) as it bakes. All the raisins in the loaf are moving away from each other at the same rate of expansion, and relative to any raisin, the raisins farther away from it would be moving away faster.
posted by haricotvert at 8:56 AM on December 5, 2014
posted by haricotvert at 8:56 AM on December 5, 2014
But now we know the entire loaf is encoded in the crust, right?
posted by haricotvert at 8:59 AM on December 5, 2014
posted by haricotvert at 8:59 AM on December 5, 2014
Well - we theorize it from an information theoretic principle, but I don't know if we can say we "know" in the same sense we know E=MC^2 or even that black holes evaporate.
Not that I don't love that idea. I think theoretically it's a fun idea to mess with, and I know we have those laser interferometers testing things regarding this, but I haven't followed up on the latest news on that.
(says someone who occassionaly reads books on this and again - would prefer if a real physicist chimed in).
Speaking of - physicsmatt, you say the current universe fits a "deSitter universe" description, but it sounds like the tecnical definition of a deSitter Universe is one that doesn't contain Mass/Energy (matter) and thus requires mostly empty space? The wiki article makes it sounds like early/pre big-bang universe was deSitter space, and that eventually via expansion, the ... well the matter will be "diluted" enough such that the effects of space expansion will be the dominant force and thus a real deSitter space again, whereas we're not technically in one now due to the local and networked effects of mass on the fabrice of space time??? Is that reading correct?
posted by symbioid at 11:11 AM on December 5, 2014
Not that I don't love that idea. I think theoretically it's a fun idea to mess with, and I know we have those laser interferometers testing things regarding this, but I haven't followed up on the latest news on that.
(says someone who occassionaly reads books on this and again - would prefer if a real physicist chimed in).
Speaking of - physicsmatt, you say the current universe fits a "deSitter universe" description, but it sounds like the tecnical definition of a deSitter Universe is one that doesn't contain Mass/Energy (matter) and thus requires mostly empty space? The wiki article makes it sounds like early/pre big-bang universe was deSitter space, and that eventually via expansion, the ... well the matter will be "diluted" enough such that the effects of space expansion will be the dominant force and thus a real deSitter space again, whereas we're not technically in one now due to the local and networked effects of mass on the fabrice of space time??? Is that reading correct?
posted by symbioid at 11:11 AM on December 5, 2014
I'm always fascinated by discussions of cosmology. At the same time they make me vaguely uncomfortable because what't the point of the struggle if there's no purpose to the universe? On the other hand it's also kind of liberating to not have a reason for being because you just are.
posted by ob1quixote at 11:33 AM on December 5, 2014
posted by ob1quixote at 11:33 AM on December 5, 2014
Well, there's no purpose to the universe of physics, but that's all just a model (or, more accurately, a whole bunch of models). An incredibly useful model, but it's not the way things "really are". It's a kind of net cast over reality through the expedient of measurement -- and again, there's no question this is an immensely practical way of proceeding -- but most people mistake the resultant model for the real thing, and wind up asking questions like "What's the purpose?" which make no sense in the context of the model. "What's the purpose?" is an infinitely regressive question in the context of a hypothetical world of discrete objects and forces interacting with each other causally in space and time, because whatever purpose you came up with, there would then have to be a purpose FOR the purpose, and so on. But that just means that if you want to know why you're here, physics (or any kind of materialism) is the wrong model -- not that there's no purpose or that the universe actually is material.
That said, it is indeed liberating to realize that the physical model admits of no purpose as we habitually think of purposes. Definitely takes the pressure off! Then you can sit back and enjoy the stars...
posted by haricotvert at 12:46 PM on December 5, 2014 [1 favorite]
That said, it is indeed liberating to realize that the physical model admits of no purpose as we habitually think of purposes. Definitely takes the pressure off! Then you can sit back and enjoy the stars...
posted by haricotvert at 12:46 PM on December 5, 2014 [1 favorite]
symboid, oops yes, you are correct. Right now we're not a perfect de Sitter because there's still matter and radiation density around. Asymptotically, we will approach perfect de Sitterness in the far future (modulo quantum fluctuations and other mysteries of the Universe we don't understand today). I got a bit sloppy and called our present Universe "de Sitter" since that's often how I refer to it in day-to-day work (since we're usually referring to asymptotic properties at that point). A certain laziness of language is surprisingly common in my line of work (figuring out when you can be lazy and when you can't is part of the grad school process).
Usually I remember to de-lazify when writing these things up here, or at least sneak it by you all with razzle-dazzle when I don't. Damn you for noticing (puts symboid on The List).
posted by physicsmatt at 4:15 PM on December 5, 2014 [1 favorite]
Usually I remember to de-lazify when writing these things up here, or at least sneak it by you all with razzle-dazzle when I don't. Damn you for noticing (puts symboid on The List).
posted by physicsmatt at 4:15 PM on December 5, 2014 [1 favorite]
The Most Massive Young Galaxy Cluster Ever Discovered
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posted by homunculus at 3:49 PM on January 1, 2015
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