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!
Reading the above I couldn’t help but imagine the aging universe developing cataracts…
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.
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.
The OP may be talking about cold spots.
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.
Nope, I did not know about this. Interesting though!
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.
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.
Assuming the edge of the universe was moving away from our point at that time, yes.
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.
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”.
What I don’t get is how a mass of stuff packed to infinite density can emit light that hasn’t reached us yet. And if we’re observing things after 13.7 billion years, how far away was the particle when it emitted the photon? It’d have to be less than 13.7 billion years, right?
I’m confused.
As the photon is emitted, and starts to travel, the space gets stretched. Ergo, there is more distance for the photon to travel, thus it takes longer to get here.
How far away was the particle that emitted the photon? Far enough away that the space could grow before the photon reached us.
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.
To be fair, we don’t call it “the thing”, we call it “the fifth thing”. Much better :).
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!
Let’s try the old-fashioned analogy.
You’re standing in the bed of a pickup truck. You throw a ball to me. That takes x time. Now put the truck into gear and get it moving. That ball will take longer to reach me because it started at a distance. Now imagine that in addition to the distance, the actual space itself is growing larger while the ball travels through it. That will add even more time that the ball needs to travel.
Inflation is like that moving truck. Any photon that was given off in our direction in the first moments when photons could be given off (about 300,000 years after inflation) is starting from a distance. No matter what that distance was, that photon has to travel farther than that initial distance because space has expanded since then. The total of the starting distance plus the increased space is 13.7 billion light years.
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.
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 10[sup]78[/sup].
Hit quote to see how to code a superscript.
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?