… or at least how much? The physics threads here got me more interested in learning about how our universe works, so I’ve been watching a lot of PBS Spacetime and How The Universe Works. According to what I’ve seen, we’ve essentially got the universe figured out to how it behaved at 10^-32 seconds after the Big Bang. Shouldn’t that tell us how much total “stuff” that emerged; i.e. energy and mass? If so, can’t we just subtract the total that we can observe from that original total?
Aside: what do the SDMB experts think about those shows? I feel like Spacetime is more precise and tries to connect everything to relativity while HTUW is more simplistic and skirts actuality in favor of simplicity on occasion. On the latter, I generally find the scientists to be entertaining and accurate. The moderators are sometimes obviously just voices. One example: “temperatures on Mercury can fall to -300 degrees, three times colder than any place on earth.” No mention of which temperature scale, and is -2 degrees “twice as cold” as -1 degree? :dubious:
I thought scientists were already pretty sure how much mass and energy were in our universe.
As far as seeing beyond our universe, you’d have to conduct experiments that proved the existence of something beyond our universe. To my knowledge this hasn’t been done yet. In part because most everything in our universe is confined to our universe. Some things like gravity can supposedly escape our universe, which could be used to experimentally verify them. I have no idea what studies have been done on it though.
Perhaps I missed something basic, but I’m assuming that the entire universe, observable and unobservable, all emerged from the Big Bang. It’s just that with expansion, much of that is moving away from us faster than the speed of light so we cannot observe it due to the cosmic speed limit. But if the know the size and temperature of the whole thing at time zero plus a minuscule amount, don’t we know the total mass and energy of all of it in aggregate?
We know something about the density of the Universe at any given point at time (not as much as we’d like to know, but the information is at least theoretically accessible). But we don’t and probably can’t know anything about how much stuff there is in total, and it may well be infinite.
We assume that the density of the unobservable Universe is the same as for the parts we can see. But how much “there” is there?
Well, yes/no/maybe so. There have been attempts to estimate the mass/energy of the universe based on our observations. However, there are a lot of slightly “off” phenomena which imply we’re missing a lot of stuff, and/or that a great deal of stuff we do see doesn’t quite work the way we expect. Truth be told, there’s a lot of open questions in physics that we can’t realistically test, as well as our thus-far-inability to completely get all our physics theories in sync.
It is my understanding that in the far future (billions, likely trillions of years from now) life that evolves in other galaxies will lose the ability to see galaxies outside their own since those galaxies will be moving away faster than light speed. So they will think their galaxy is the only one that exists, while we know there are hundreds of billions of them.
Lawrence Krauss said something similiar to what you are saying, but I don’t know if it is verified. If there are things ‘within’ our universe that we can’t see because they are moving away from us too fast. I was under the impression that since our universe is only ~14 billion years old, there wasn’t anything moving that fast yet.
Inflationin the early universe expanded spacetime at many times of the speed of light (which affects only matter, not space). That’s the cause of having material today that is receding too fast to be seen.
It’s not clear what’s meant by saying that A is “three times colder” than B. It would be much more clear to say B has three times as much heat as A. This could be measured on any scale where zero is absolute zero, the point at which there is no heat at all. The coldest recorded place on Earth (natural weather only) is −89.2 °C , or −128.6 °F, or 184.0 Kelvin. One third of 184.0 K would be about 61.3 K, which is -349.3 degrees F. It is fair to say that 184 K is three times as hot as 61.3 K, hence 61.3 K could be said to be three times as cold as 184 K.
Right - no physical objects are moving faster than light, because it’s impossible to accelerate mass up to c, or exceed c, because the faster you go, the more energy is required to go even faster (and the more mass the object gains due to moving faster, until at light speed you would effectively have infinite mass and require infinite energy to continue accelerating.
However, the fabric of spacetime, being massless, is not subject to the otherwise universal light-speed restriction. In the same way the theoretical Alcubierre drive would entail creating a bubble of warped space around a starship, expanding the space in front of it and behind it faster than light; while the ship itself actually remains stationary or nearly so at the center of the bubble and does not travel faster than light, but is carried along. The universe is doing largely the same thing, with the galaxies sitting on the fabric of spacetime being pulled away from each other.
Given the combined effect of space expanding in both directions, and having continued to accelerate in its rate of expansion since the beginning of the universe as was confirmed in the '90s, if you point a telescope at the most distant observable objects, their redshift will make it appear that the objects themselves are traveling faster than light. They’re not - the combined motion of the objects, and the fabric of spacetime expanding, gives the apparent result of the objects going FTL.
Willing to bet I’m a bit off on a couple minor details since I’m just an enthusiast but that’s the gist.
Can I suggest reading the book We have no idea by Jorge Cham (of PHD Comics) and Daniel Whiteson, Professor of Physics at UCI.
The question about the size of the universe, and what’s beyond the limits of our observation, is discussed in detail in chapter 15.
This is the best overview of the current state of physics that I’ve seen, and in a very light, entertaining, and readable format.
In spite of its light tone, this a serious book about serious physics, incorporating the latest insights and experiments from the past few years. It’s not sensationalist or superficial. It’s very thought-provoking. The emphasis is on what we don’t know. If you ever thought that modern physics has a pretty good idea about how the universe works, this book shows how many fundamental scientific questions still remain unexplained. We’ve barely scratched the surface.
Thanks for the recommendation, looks like a fun read.
And yeah, we’re missing quite a lot - when 80-90% of all the mass in the universe is unaccounted for, gravity behaves differently on large scales (outside / between galaxies and superclusters) than it does on small (stellar / inside galaxies) ones, and on even smaller (quantum) scales objects behave significantly differently than on a macro scale… at some point all the theories that try to account for each of the problems themselves become problems. It won’t be until the next Einstein comes along and points out the essential piece of the puzzle that everyone is missing, that most of the mysteries resolve themselves and we realize what theories were correct and which were invented simply to try to explain what we couldn’t.
Not both of these statements are true. If dark energy is “stuff”, then gravity behaves the same at all scales, and it’s just that we don’t understand the nature of most of the stuff. If, on the other hand, dark energy is really a cosmological constant, then gravity does behave differently on large scales, but the stuff that we understand is a larger (though still minority) component of the total stuff.
The simple answer is ‘we can’t observe it, so how could we know about it’? It really is that simple, we can only take measurements of a “limited” volume of space and anything beyond that is just plain unknown (it seems silly to call a sphere with a radius of 46 Billion light years limited, but that’s space for you). And because of limitations of expansion and the speed of light, there’s a limit past which nothing is even theoretically observable. We can take guesses at what’s out there, we can put some limits on it by seeing if there appear to be unexpected effects on what we can observe, but there’s really no way to get a firm answer. Currently the ‘best guess’ is that the universe is actually infinite, but that’s not something that could ever be proved by observation.
You’re making the mistake of thinking of the Big Bang as an explosion of matter from a single point of space into space. The Big Bang denotes a point in time, not a point in space, happened everywhere (or at least throughout the observable universe), and involves the expansion of space, not what we’d typically consider an explosion. We know the density of energy at the time of the big bang, but what is now the observable universe was about the size of a grapefruit at 10^-32 seconds, and we don’t know what is outside of it. We assume that however much more stuff there was looked basically the same as the part we can see, but we don’t know that for sure and don’t know how much more stuff would be involved.
If something happens right… NOW a million light years away, it can’t affect us for a million years. We can’t even know that it happened for a million years. Both things are happening at the same point in time, though. Time is a separate dimension from space.
That’s just as true for the early universe. Whatever caused inflation affected each individual point in space* at a single point in time. Not a point *inside *space, but every point *of *space. That’s the critical distinction.
*My language crudely indicates that space has individual points, i.e. is quantized, which may or may not be true. The concept remains true in either case.
The reason we don’t know what’s beyond the observable universe is that we can’t observe it.
Is this a trick question?
I mean, there’s no reason to believe that it isn’t the same sort of stuff we observe in the observable universe, so it is very likely just like what we see. But maybe not. But we will never know because you can’t get there from here.