A part of the universe is opaque. Why?

[blood63]

Why can we only see a part of the universe? As the opaque part covers moves further apart and occupies more volume, will it eventually become transparent?

Also, how fast is the universe expanding? Not at the speed of light, right? So the universe is older than the 13.7 billion years which is its size in light years.

Thanks!

[sevenwood]

Originally Posted by blood63

As the opaque part covers moves further apart and occupies more volume...

Reading the above I couldn't help but imagine the aging universe developing cataracts....

[Candyman74]

The further away something is, the longer it takes for its light to reach us. When you see something 13.7 billion light years away, therefore, you're seeing it as it was 13.7 billion years ago. Since 13.7 billion years is the age of the universe, you're seeing the earliest possible things in the universe's history.

At that time, the universe was a hot, opaque, dense plasma thingy. It took time to cool and expand and become tranparent. So if you look far back enough (i.e. far enough away), that's what you would see.

Anything further than 13.7 light years away, you can't see, because its light hasn't reached us yet. And since the universe is expanding faster than the speed of light*, you'll never see it because it's moving away faster than the light can move.

*The expansion of space isn't limited by the cosmic speed limit. That's because it's not something "moving in space" (which you can only do up to speed c) but it's space itself expanding. The speed at which something recedes from us, therefore, is larger and larger the further it is away, because there is more space between us and it to expand. When you double the size of a small thing, it moves a small amount, and so its far edge hasn't moved fast. When you double the size of a large thing in the same amount of time, it moves a large amount, and so its far edge recedes faster.

[Exapno Mapcase]

Cosmic dust and gases make up much of the universe - and even much of our own galaxy - opaque to visual spectrum telescopes. Most of it can be penetrated by using other frequencies but not all.

Expansion may eventually thin these particles out to nothingness, but that's over so long a scale that what is opaque at that time has no meaning.

The age of the universe is 13.7 billion years, so that the radius of the observable universe is also 13.7 billion light years. The universe is thought to have a total radius of 46 billion light years due to the faster-than-light expansion of space. Total size and age are not the same number; age and observability are the same number.

[blood63]

Originally Posted by Exapno Mapcase

Cosmic dust and gases make up much of the universe - and even much of our own galaxy - opaque to visual spectrum telescopes. Most of it can be penetrated by using other frequencies but not all.

Expansion may eventually thin these particles out to nothingness, but that's over so long a scale that what is opaque at that time has no meaning.

The age of the universe is 13.7 billion years, so that the radius of the observable universe is also 13.7 billion light years. The universe is thought to have a total radius of 46 billion light years due to the faster-than-light expansion of space. Total size and age are not the same number; age and observability are the same number.

My thick skull is struggling to get this.

The edge of the observable universe is 13.7 billion light years away. That means that light takes 13.7 billion years to reach us from there. If the universe expanded at the speed of light it would take 13.7 billion years for it to move that far away.
BUT the universe is expanding faster than the speed of light (I don't get that at all) so it would take less than 13.7 billion years to get that far away.
BUT the universe is speeding up, accelerating. Classical physics tells me that after the initial force, the object moves at a constant speed unless acted on by an opposing force. What external force is causing it to move faster?
BUT, the universe is 40 billion light years in radius. That tells me the universe actually ballooned to 40 billion ly in 13.7 billion years. That makes it a bit easier to visualize.

MAYBE I'm trying to see it all in 3D space and maybe that is not possible.
But there is more space beyond it that we can't see.

[Exapno Mapcase]

Most scientists believe that the universe went through a period of inflation in its first few instants. That expanded space much faster than the speed of light, forever driving some of its contents out of causal connection to our observable universe.

After inflation, the universe kept expanding but at less than the speed of light. We can only see out as far as 13.7 billion light years, but there's more stuff beyond that we can't see. Remember, though, that the universe is everything. The universe doesn't expand into empty space or any other thing. It just expands.

We expected that the stuff that started moving away from us during inflation would be slowed over time by gravity. That has recently been proved wrong. The rate of movement is increasing rather than decreasing. We don't know exactly why. It can't be an external force - it must be an internal one. There is nothing external to the universe. And this is a measure of stuff moving inside of space, not of space itself expanding. That movement is limited to the speed of light.

[Irishman]

Originally Posted by blood63

The edge of the observable universe is 13.7 billion light years away. That means that light takes 13.7 billion years to reach us from there. If the universe expanded at the speed of light it would take 13.7 billion years for it to move that far away.

Assuming the edge of the universe was moving away from our point at that time, yes.

BUT the universe is expanding faster than the speed of light (I don't get that at all) so it would take less than 13.7 billion years to get that far away.

Suppose the universe is a sheet of elastic, with dots drawn on it for stars (or galaxies or whatever). Now use a pen and start drawing a line from one star to the next one. As you move the pen, stretch the elastic.

The movement of the pen drawing the line is the movement of the photons of light. The stretching of the elastic is the expansion of space. As you can see, the two are not related. One can move the pen very slowly and stretch the elastic very quickly, and the distance between two dots will grow faster than the movement of the pen.

BUT the universe is speeding up, accelerating. Classical physics tells me that after the initial force, the object moves at a constant speed unless acted on by an opposing force. What external force is causing it to move faster?

Answer that and win the Nobel Prize. Calling it "dark energy" is just labeling our ignorance. It's fundamentally not much better than calling it "the thing" or "whachamacallit".

[The Hamster King]

Originally Posted by blood63

My thick skull is struggling to get this.

Stop thinking about the Big Bang as an explosion.

The universe pops into existence. It's already infinite in size. And its packed full of hot, dense plasma.

It starts to expand. It's not getting any bigger (it's already infinite), but things inside it are moving apart. It's becoming less dense.

Eventually, after a few hundred thousand years the fog clears. The density drops low enough that light can actually travel a long distance without getting reabsorbed.

It takes time for light to travel a long distance. We can see that first bit of light that shone out when the fog cleared, but only from fog that was very, very far away. All the light from the nearby fog has already moved past us.

As the light from the distant fog moved toward us, space continued to expand. So the furthest region of space we can see now (where that distant fog used to be) is much further away than it was when the light was emitted.

And as that light has been traveling toward us, it's been stretched by the expansion of space. So instead of being white-hot, it's now a dim microwave glow.

[Tom Tildrum]

The OP may be talking about cold spots.

[JohnGalt]

Isn't there a theory out there that if it weren't for cosmic dust, our night-time sky would be very bright - that the band we see as the Milky Way would be as bright as a full moon? Are there any simulations or visual representations of what that might look like?

[blood63]

Originally Posted by Tom Tildrum

The OP may be talking about cold spots.

Nope, I did not know about this. Interesting though!

[Candyman74]

Originally Posted by JohnGalt

Isn't there a theory out there that if it weren't for cosmic dust, our night-time sky would be very bright - that the band we see as the Milky Way would be as bright as a full moon? Are there any simulations or visual representations of what that might look like?

Yeah, a lot of our view is obscured by dust and the like.

[Chronos]

13.7 billion years is the age of the Universe. The size of the visible Universe is a fair bit bigger than that, since the most distant things we see have gotten even more distant in the time since they emitted the light we're seeing.

Answer that and win the Nobel Prize. Calling it "dark energy" is just labeling our ignorance. It's fundamentally not much better than calling it "the thing" or "whachamacallit".

To be fair, we don't call it "the thing", we call it "the fifth thing". Much better .

[tdn]

If the universe is expanding, and we are in and of the universe, does that mean that we are expanding as well? I'm trying to reduce!

[D18]

I've got nothing to add, except to further boggle the minds of the boggled, (or at least this boggled my mind when I first learned it), in the inflationary period, the universe achieved the size of a grapefruit. That was, however, an expansion of a factor of 10-to-the-26th (sorry - don't know how to superscript).

[Exapno Mapcase]

Originally Posted by D18

I've got nothing to add, except to further boggle the minds of the boggled, (or at least this boggled my mind when I first learned it), in the inflationary period, the universe achieved the size of a grapefruit. That was, however, an expansion of a factor of 10-to-the-26th (sorry - don't know how to superscript).

As noted in the link to inflation I posted, you have to think in terms of volume, not linear distance. So the proper factor is 10⁷⁸.

Hit quote to see how to code a superscript.

[Chronos]

If we can't see beyond the 13.7 billion light years, how do we know what's there? How do we know anything is there?
BTW, in another post, someone wrote that the universe is infinite. I always thought it was finite. Did I read that wrong?

We don't know what's out there. But everything that we can see seems to be pretty much the same everywhere (on a sufficiently large scale), and it'd be really weird if, as soon as we can't see it, it starts behaving differently. It's sort of like asking how you know that the refrigerator light goes out when you close the door: Fun as a silly question, but it can't really be serious.

And we don't know whether the Universe is finite or infinite. It appears to be flat, and the simplest models of a flat universe imply an infinite one, but it could be positively curved on scales much larger than we can see (sort of like how the surface of the Earth appears flat, even though it's not), or it could be a slightly more complicated situation that really is flat but is finite anyway. In a way, it's not controversial at all, since everyone agrees that we don't know.

[Chessic Sense]

Originally Posted by Exapno Mapcase

Let's try the old-fashioned analogy.
... The total of the starting distance plus the increased space is 13.7 billion light years.

Originally Posted by Irishman

How far away was the particle that emitted the photon? Far enough away that the space could grow before the photon reached us.

Originally Posted by Tom Tildrum

Think of the "Dolly Zoom" and a character in a horror movie running down a hallway that just keeps getting longer.

I want a number, though. How far was the fog that we're seeing now? How far is that particle now? Was it, say, 13 billion light years away and expansion has added 0.7 billion years to the trip? And now those particles are 30 billion light years away, or what?

[The Hamster King]

Originally Posted by Chessic Sense

I want a number, though. How far was the fog that we're seeing now? How far is that particle now? Was it, say, 13 billion light years away and expansion has added 0.7 billion years to the trip? And now those particles are 30 billion light years away, or what?

The Wikipedia page on the observable universe has the answers you seek.

The fog we can see now was only 42 million light years away when it emitted those photons.

But it took 13.7 billion years for those photons to reach us because they were moving "upstream" against the expansion of space.

And because of that expansion, the area of the universe where that fog used to be is now 46 billion light years away from us.

[Sister Vigilante]

Originally Posted by Chronos

We don't know what's out there. But everything that we can see seems to be pretty much the same everywhere (on a sufficiently large scale), and it'd be really weird if, as soon as we can't see it, it starts behaving differently. It's sort of like asking how you know that the refrigerator light goes out when you close the door: Fun as a silly question, but it can't really be serious.

And we don't know whether the Universe is finite or infinite. It appears to be flat, and the simplest models of a flat universe imply an infinite one, but it could be positively curved on scales much larger than we can see (sort of like how the surface of the Earth appears flat, even though it's not), or it could be a slightly more complicated situation that really is flat but is finite anyway. In a way, it's not controversial at all, since everyone agrees that we don't know.

First of all, this thread is fascinating. I can barely understand it but I'm trying.

Why do we think the universe is flat? That's the first I've heard of this.

[Exapno Mapcase]

Originally Posted by Sister Vigilante

Why do we think the universe is flat? That's the first I've heard of this.

The Wiki page on Shape of the Universe is a good place to start.

[Moe]

Originally Posted by Sister Vigilante

First of all, this thread is fascinating. I can barely understand it but I'm trying.

Why do we think the universe is flat? That's the first I've heard of this.

I'm with you on that. Fascinating, despite being largely over my head. Here's Lawrence Krauss on why he claims the universe is flat.

[Irishman]

Originally Posted by Exapno Mapcase

Remember, though, that the universe is everything. The universe doesn't expand into empty space or any other thing. It just expands.

This is the kind of statement that is completely unintelligible. It's like stating you have a round square. How can it be round and a square? The definition of square defies roundness. How can something expand and not expand into something? How can something expand without getting larger? Bdbdbdbdbdbd.

Originally Posted by Candyman74

What's infinity +1? It's still infinity.

But it's a bigger infinity.

Basically, things are moving further apart within that infinite area, so it's becoming less dense. But there is no "edge" that could be moving away from you, so there's no way you can say the construct you call the universe is getting "bigger".

It's a round square.

Originally Posted by Balance

This is just a nitpick, but might it be better to say that it expanded much faster than c?

Isn't "c" just an abbreviation for the speed of light in a vacuum? It's the constant "c" because we believe it is a constant, but if the speed of light in a vacuum were very different in the past, that term would also be abbreviated "c". The numerical value would be different, and we'd have to change our wording a little (c is no longer a constant), but it would still fit in the same formulas as c. We'd just mean a varying number instead of the constant value we currently assign.

[The Hamster King]

Originally Posted by Irishman

How can something expand and not expand into something?

What we actually observe is a decrease in density. Things in the universe used to be more densely packed and now they're less densely packed.

This is commonly represented as "the universe is expanding". But we can't see any edge. We don't even know if there IS an edge. All we know is that overall the density of the universe is decreasing.

[Exapno Mapcase]

Originally Posted by Irishman

But it's a bigger infinity.

No, it's not. Infinity + 1 is exactly the same size as infinity, because infinity is not a number. It's more of a process. The even numbers are an infinity. The odd numbers are an infinity of the same degree. The even plus odd numbers are an infinity of the same degree. The even and odd numbers plus 0 are an infinity of the same degree. The even and odd numbers plus 0 plus the negative numbers are an infinity of the same degree. This is completely settled math, not physics.

Isn't "c" just an abbreviation for the speed of light in a vacuum? It's the constant "c" because we believe it is a constant, but if the speed of light in a vacuum were very different in the past, that term would also be abbreviated "c". The numerical value would be different, and we'd have to change our wording a little (c is no longer a constant), but it would still fit in the same formulas as c. We'd just mean a varying number instead of the constant value we currently assign.

C is used in both ways in normal speak, as the actual speed of light in a vacuum and as the constant that is plugged into equations to make the units come out properly. That's normally proper shorthand because in most cases they can be treated interchangeably. This would be an exception in which they can't. If the constant we think of as c is actually a variable then a lot changes. Fortunately, I don't think that many scientists believe that c was actually a variable even during inflation.

[Candyman74]

Originally Posted by Irishman

This is the kind of statement that is completely unintelligible. It's like stating you have a round square. How can it be round and a square? The definition of square defies roundness. How can something expand and not expand into something? How can something expand without getting larger? Bdbdbdbdbdbd.

Because - as I've said many times on these boards - the fact that someone (in this case the worldwide physics community) chooses an imprecise non-mathematical term to attempt to describe something in the poorly-equipped English language doesn't then mean that you can take that term and use it as a basis for reasoning. It's just a word chosen to try and describe something which the English language can't describe well, and the further implications of that word don't apply. The word itself can't be the basis of a logical train of thought, because the word itself isn't accurate.

The correct description, unfortunately, is a bunch of math. If you want to apply reasoning to the implications or consequences of a description, that's the description you have to first fully understand and then work with. Which isn't something that's gonna happen on a messageboard thread - or anywhere outside of a very long research paper.

It sucks that the universe is so complicated and that we have to use such imprecise language and poor analogies to describe it outside of the math, but there it is.

But it's a bigger infinity.

I'm afraid it's not. Infinity doesn't work that way. If it can get bigger, then it wasn't infinite in the first place.

[Candyman74]

Originally Posted by Irishman

This is the kind of statement that is completely unintelligible. It's like stating you have a round square. How can it be round and a square? The definition of square defies roundness. How can something expand and not expand into something? How can something expand without getting larger? Bdbdbdbdbdbd.

Because - as I've said many times on these boards - the fact that someone (in this case the worldwide physics community) chooses an imprecise non-mathematical term to attempt to describe something in the poorly-equipped English language doesn't then mean that you can take that term and use it as a basis for reasoning. It's just a word chosen to try and describe something which the English language can't describe well, and the further implications of that word don't apply.

The correct description, unfortunately, is a bunch of math. If you want to apply reasoning to the implications or consequences of a description, that's the description you have to first fully understand and then work with. Which isn't something that's gonna happen on a messageboard thread - or anywhere outside of a very long research paper.

It sucks that the universe is so complicated and that we have to use such imprecise language and poor analogies to describe it outside of the math, but there it is.

We can complain that physicists make it so unintelligible - but they don't. They do their best to describe what they find in conversational language; and they don't do a terrible job of it, but that language just ain't equipped. It's not their fault.

But it's a bigger infinity.

I'm afraid it's not. Infinity doesn't work that way. If it can get bigger, then it wasn't infinite in the first place.

[iamnotbatman]

Originally Posted by Irishman

This is the kind of statement that is completely unintelligible. It's like stating you have a round square. How can it be round and a square? The definition of square defies roundness. How can something expand and not expand into something? How can something expand without getting larger? Bdbdbdbdbdbd.

If it helps, think of the universe like a chess board. There are 8x8 squares. Suppose we increase the "size" of the board to 16x16. We don't have to make the board "bigger". We can just make each square "smaller." In fact, you could make the chess board any size you like; it really doesn't matter. When you start thinking about these things carefully you realize that all that matters is the number of squares; other than that the "size" of the board is meaningless unless it is relative to something.

[Candyman74]

Originally Posted by Sister Vigilante

First of all, this thread is fascinating. I can barely understand it but I'm trying.

Why do we think the universe is flat? That's the first I've heard of this.

It doesn't mean flat as in a 2D sheet of paper. It means flat in terms of the curvature of space time.

[Chronos]

Around this point, one should also caution that there are different ways to define distance on cosmological scales, which can yield different values. Not that I'm speaking not of different ways of measuring distance, but different interpretations of what the concept of distance actually is.

[romusz]

Regarding the speed of light...wouldnt that be proof that "ghosts" exist? If youre just now seeing an object that has been "dead" for millions of years...thats proof enough, youd think?

[Chronos]

Only if you're defining "ghost" completely differently than most folks do.

[Autolycus]

Yea that reminds me of the episode of Scooby Doo where they pull off the mask of the guy whose been pulling all the stunts in the haunted circus and it turns out the perpetrator was the Crab Nebula all along.

[Northern Piper]

A part of the universe is opaque. Why?

It's the stars' changing room.

[Candyman74]

Originally Posted by billfish678

I seem to recall there are different kinds of infinities.

You'll have to expand on that, then, because I don't know what you mean. I mean, there's the mathematical idea if infinity, and there's probably a million different ideas from what different people think it means, and Hollywood infinities, and religious infinity, but in this context we're talking about the mathematical concept.

[ZenBeam]

The infinity of the reals is a bigger infinity than the infinity of the integers. There are an infinite set of infinities of different sizes.

I don't think that is really relevant to your point, though. ETA: In particular, when Irishman said "But it's a bigger infinity", it wasn't. It was the same size of infinity.

[Candyman74]

Originally Posted by Irishman

Agreed. But that is why non physicists have such a hard time with this stuff. These words come packed with all sorts of inherent assumptions about what they mean and how they work. When a scientist uses a specialized use of the word to convey one element of the concept they are trying to express, it is only natural for the non-specialist to try to extend the concept with all the analogies that come with those words. That is how communication usually works. So when it doesn't work, it is exceptionally frustrating, and thus leaves the non-specialist think the scientists must be full of it, or are pulling a big prank on the rest of us.

Yup.

So part of the burden for the scientist trying to use these words to laypeople to explain what they are talking about is the burden to caveat that the words being used are fuzzy approximations to drag out one element of what they mean to convey, and not intended as direct analogues that one can extrapolate with. They must try that much harder to make their examples and analogies and descriptive models limited and precise. Because the layperson cannot be expected to know what you mean by "expanding" when what you mean is not getting larger, but getting less dense.

I guess if they're selling you a book or something, then they have that burden. And if they don't fulfil it to your satisfaction, you vote with your wallet.

Other than that, their burden is only to get on with their research and do useful stuff.

Or, if they're just taking personal time to explain stuff to you on a messageboard, they have no burden at all. They make what effort they feel like.