10^80 particles in observable v unobservable universe

robertjford80

Here is a chart showing the unobservable universe



The green lines indicates the part we can see. My question is that figure that gets thrown around, there are 10^80 particles in the universe, is that for observable or the unobservable universe.

nicksauce

10^80 is for the observable universe, as the unobservable universe is probably [maybe?] infinite.

robertjford80

that the unobservable universe is not infinite is something i feel very strongly about. it's quite simple: the universe began at a definite time in the past and it is bounded by the present, so there is a boundary on the past and the present, hence finite. Second argument: in an infinite universe there is an infinite amount of matter which would overwhelm the cosmological constant thus causing the universe to implode into a big crunch. third, you can't do any measurements in an infinite universe because all quantities are infinite, thus there is no way to determine if space is flat. fourth, although this is an argument from authority, take a look at the graph, it clearly shows the unobservable universe to be not infinite but finite at roughly 46 billion light years. fifth, you can't calculate probabilities in an infinite universe, the odds of all events happening are infinite. sixth, as olbert demonstrated in the 1830's in an infinite universe the sky would be filled with light from every angle.

nicksauce

A timelike boundary does not imply a spacelike boundary.

Plainly false. The Friedmann equations show that the big crunch only results in universes with positive curvature (i.e., finite universes)

Just do your measurement in some local patch. Why does the rest of the universe matter?

You are way off-base here. 46 Glyr is the size of the **observable** universe.

I don't think this makes any sense.

This is true for an infinitely old universe, but is solved by a finite age universe. If the universe is of finite age, there we can only have received light from so far away, so the sky will not be filled with light from every angle, regardless of whether or not the whole universe is infinite.

robertjford80

Sure it does. Go back to first year physics. Take the following equation:

D = RT, distance equals rate times time. You're arguing the D is infinite. How do you get D to be infinite? Either R or T has to be infinite. What's the R of the universe? It's the Hubble constant, 75 km/s/MPc. What's the T of the universe. It's 13.75 by. Hence D is finite.



That's ridiculous. I have a feeling that you're talking about the multiverse for which there is no evidence. It's the universe as a whole that matters. Let's say my local patch was the solar system and someone else's local patch was the system of Alpha Centauri, clearly our measurements would be in disagreement.

Look at the chart again. It's that curved green line that represents the observable universe.

What is the probability that I'm a random word generator? How do you determine the probability of something? You take the number of events divided by the probability space. in an infinite universe there is an infinite number of random word generators, moreover, the space in which it can happen is infinite, so you can't determine the probability of something happening.

DaveC426913

Yes you can.

While it is probably true that, in an infinite universe, all things that can happen will happen, so what? our cause and effect is still limited to our own light cone.

Show me one example where something that happens 10³⁰ light years away has any effect on our calculations or physical universe here.

phinds

Yes, but the universe really doesn't care what you think, and as nicksauce has pointed out, your logic is seriously flawed.

Your inability, or unwillingness, to accept that the universe might have STARTED OUT infinite (weird as that does seem), is your limitation, not the universe's. Just because it was a lot smaller 14+ billion years ago does NOT imply that it was finite.

robertjford80

Probability at least can be debated. The following however cannot:

Take the following equation:

D = RT, distance equals rate times time. You're arguing the D is infinite. How do you get D to be infinite? Either R or T has to be infinite. What's the R of the universe? It's the Hubble constant, 75 km/s/MPc. What's the T of the universe. It's 13.75 by. Hence D is finite.

I would like an answer to the above.

robertjford80

I'm not saying: I feel strongly about X therefore X is true. I'm saying that X is true, I think it's important. A lot of people think it's false, and I care deeply about this misconception.

For you as well, phinds, I would like an answer to this:

D = RT, distance equals rate times time. You're arguing the D is infinite. How do you get D to be infinite? Either R or T has to be infinite. What's the R of the universe? It's the Hubble constant, 75 km/s/MPc. What's the T of the universe. It's 13.75 by. Hence D is finite.

This is just a bald assertion. You've got no reason for this statement of faith.

DaveC426913

I'm not. I'm simply poking holes in the flawed logic that's been presented.

DaveC426913

As is yours. "It is finite" is a statement of faith. You can't apply things like D=R/T to the universe.

More correctly, we think it's probably finite.

Number Nine

This betrays an ignorance of basic probability. You can certainly calculate probabilities that way when dealing with a finite sample space, but calculus allows us to deal with infinite (or uncountable) probability spaces very easily. The normal distribution itself is, of course, defined over the reals...

You're assuming here that the universe began with radius zero and that expansion involves the increase in radius over time, but this isn't true. Expansion, in the context of the bang bang, refers to metric expansion, which is very different.

Dickfore

Your equation is dimensionally incorrect.

robertjford80

You're going to have to show how metric expansion implies an infinite space.

Also an infinite universe conflicts with standard inflationary cosmology. Here's a quote from Lisa Randall

You can't double the size of the universe if it is infinite.

I also want to get back to the density parameter. You can't calculate the density of an infinite universe. Such statements as the universe is 4% ordinary matter, and 24% dark matter are meaningless because in an infinite universe there is an infinite amount of matter.

robertjford80

You're just asserting that you can't apply things like D = RT to the universe. You're going to have to provide some reason why our common sense intuitions should be suspended. If you look at the chart above, it clearly asserts that the size of the universe is 46 billion light years. It doesn't require faith to believe that a finite number times a finite number equals a finite number.

If it's more probable that the universe is finite then we're in agreement. Of course there's always a small probability that the universe is infinite but that's not what we're arguing and that's not what nicksauce asserted.

Number Nine

No, you're completely missing the point here. Your argument was as follows: the expansion of the Universe involved the Universe beginning with finite size and increasing in radius over time, therefore the current size of the Universe must be finite. You grossly misunderstand the notion of inflation in Big Bang cosmology; and your argument fails utterly when expansion is understood as metric expansion.

Brilliant! It's incredible that a century's worth of physicists and mathematicians have simply overlooked this fact. If only we had a mathematical framework that allowed us to deal with infinite processes; it's a shame that Isaac Newton didn't think of one...

No, it doesn't. That chart very obviously describes the radius of the observable universe as 46 light-years. It even has a nice caption saying "visibility horizon" above the bar reading 46 light-years. How have you failed to notice this?

Dickfore

Hey OP, imagine a (infinite) plane. Next, imagine the plane stretching evenly. If you drew two points on it, they would recede from eachother. So would any two arbitrary points. Did the size of the plane increase? Next, imagine two distant points and a light signal emanating from the first one towards the second one. If the points are sufficiently far apart, the distance between them would increase by more than the distance traverseed by the light. In this fashion, we arrive at an observable region around each point. Does this region increase?