The time it takes for light to travel back to earth

syngindub

Is it true we can see the remnant's of the Big Bang from Static on tv?

Capt'n Midnight

syngindub wrote: »

Is it true we can see the remnant's of the Big Bang from Static on tv?

no.


well yes, but it's only a tiny fraction of static on the screen

on very old TV's most of the noise was generated internally to prevent a bright spot burning the screen if there was no signal

Standman

It's all such mind bending stuff! Is there an 'edge' of the universe, i.e. a point after which space ceases to exist?

ps200306

Standman wrote: »

It's all such mind bending stuff! Is there an 'edge' of the universe, i.e. a point after which space ceases to exist?

There's a number of different "edges" (all in theory ... we can't actually see any of them, although we're pushing the boundaries back with technology).

First of all, when we look out we are looking back in time. In principle we could look all the way back to the Big Bang, and this would be the ultimate "edge" of space and time.

In practice, the universe was full of a hot plasma until between 300 thousand and 400 thousand years after the Big Bang. Like the plasma that the sun and stars are made from, it was opaque to light. When it cleared (gradually, like a clearing fog) light was set free to travel the universe. So we can't see back any further than this (although for the reasons given later, we can't see this far either). This "edge" is commonly called the "last scattering surface". A common mistake is to think that this is where the cosmic microwave background comes from, since it represents the time at which the CMB was set free. But the CMB comes from all around us, reminding us that our spot in the universe, along with every other spot, were once squeezed together into a tiny volume.

Yet another "edge" is the one from which a ray of light which has been travelling toward us since the beginning of time would reach us. This could be called the edge of the observable universe, but the cosmologists use the fancy name of "the particle horizon". As you will realise, the particle horizon is not a fixed distance away -- new stuff is coming into view all the time as the universe gets older. The particle horizon is currently about 46 billion light years away.

One of the most astounding discoveries of the last two decades is that the rate of expansion of the universe slowed for ten billion years after the Big Bang under the force of gravity, but is now accelerating again due to "dark energy"! So, although new things are coming into view over the particle horizon, eventually this will stop happening, and in fact things will start to disappear again in future. So there is a maximum limiting distance that we will ever be able to see, and this "edge" is called the "cosmic event horizon". It's about 60 billion light years away.

So there you have it -- four "edges": the Big Bang itself, the last scattering surface, the particle horizon and the cosmic event horizon.

(Just in case you're wondering how we can see stuff 46 billion light years away when the universe is less than 14 billion years old ... well, it's complicated: unfortunately in an expanding universe there are multiple ways to measure distance none of which has an exclusive claim on being "right").

jacksie66

This post has been deleted.

orchidsrpretty

Can the universe be likened to being inside an ever expanding sphere? Is this why it said space is curved?

RebelButtMunch

Standman wrote: »

It's all such mind bending stuff! Is there an 'edge' of the universe, i.e. a point after which space ceases to exist?

Sometimes a question itself is not valid. For example. "What happens if your at the north pole and you go north?"

So maybe the idea of the universe having an 'edge' isn't a correct assumption. It's not easy to get your head around that.

ps200306

orchidsrpretty wrote: »

Can the universe be likened to being inside an ever expanding sphere? Is this why it said space is curved?

Not quite inside the expanding sphere -- this would imply a boundary surface that we could travel to in principle. It is probably better to think of the surface of an expanding balloon with dots on it that are moving away from each other as the rubber between them expands. In this analogy, the two-dimensional surface of the balloon represents 3-D space. Even if the surface is finite, so that you could travel around it, it has no boundary as such. There is no concept of "inside the balloon". All of space exists on the surface (a so-called hypersurface in the real world of 3-D space).

One implication is that if you could see far enough in opposite directions you'd be able to see the same stuff from opposite sides. (Remember that light is constrained to follow the surface of the cosmic balloon too). Scientists have looked for that in the CMB maps of the sky, but have found nothing. So if the universe is a hypersphere, the radius of curvature is bigger than the observable universe.

However, a hyperspherical geometry is only one possibility -- one with so-called positive curvature. It is also possible to have negative curvature, in which case the 2-D analogy would be a hyperbolic surface, shaped like a saddle (nowadays the analogy used more often is a Pringle :pac:) that curves away infinitely in all directions. In between these extremes there are more or less curved hypersurfaces, with the zero-curvature option being "flat". Positive, negative, and zero curvatures are associated with universes that will collapse in the future, or expand without limit, or will just coast along asymptotically -- expanding more and more slowly but never quite stopping. Or, at least that was the case when we thought everything was governed by the matter density of the universe and its curvature. Nowadays we also have dark energy to deal with. But that's another story. We now think we're in a universe that will expand forever, but the geometry seems extremely close to flat.

King George VI

RebelButtMunch wrote: »

Sometimes a question itself is not valid. For example. "What happens if your at the north pole and you go north?"

So maybe the idea of the universe having an 'edge' isn't a correct assumption. It's not easy to get your head around that.

There are two logical possibilities. I'm sure there are more that I'm not aware of but the ones I think make the most sense are:

1. The universe has an edge - a point where the universe just kinda stops.

2. It just goes on infinitely.

Either of those explanations seem equally ridiculous to me when trying to picture it.

Standman

ps200306 wrote: »

One of the most astounding discoveries of the last two decades is that the rate of expansion of the universe slowed for ten billion years after the Big Bang under the force of gravity, but is now accelerating again due to "dark energy"! So, although new things are coming into view over the particle horizon, eventually this will stop happening, and in fact things will start to disappear again in future. So there is a maximum limiting distance that we will ever be able to see, and this "edge" is called the "cosmic event horizon". It's about 60 billion light years away.

Wow! I had heard about dark energy accelerating the rate of expansion but thought that had been happening from the start! It makes it so much more intriguing that it would just suddenly start taking effect ~3 billion years ago for no apparent reason. Are there any theories as to what could have possibly brought dark energy into existence at that point in time?

Thanks for the lengthy reply by the way, all fascinating stuff!

Standman

St. Jimmy wrote: »

There are two logical possibilities.

1. The universe has an edge - a point where the universe just kinda stops.

2. It just goes on infinitely.

Either of those explanations seem equally ridiculous to me when trying to picture it.

Well it could be possible that if you keep going in a straight line you could end up back at your starting point in space. In this case it would be an infinite loop just like circumnavigating the globe. On earth, if you want to leave it you can't just keep going north, you have to go up. So possibly in space, if you want to go 'leave it', then you have to go in another direction that isn't up, down, forward, backward, etc. That would be impossible for three dimensional beings like us!

King George VI

Standman wrote: »

Well it could be possible that if you keep going in a straight line you could end up back at your starting point in space. In this case it would be an infinite loop just like circumnavigating the globe. On earth, if you want to leave it you can't just keep going north, you have to go up. So possibly in space, if you want to go 'leave it', then you have to go in another direction that isn't up, down, forward, backward, etc. That would be impossible for three dimensional beings like us!

I guess you're right. That's where a Stargate comes in handy.

ps200306

Standman wrote: »

Wow! I had heard about dark energy accelerating the rate of expansion but thought that had been happening from the start! It makes it so much more intriguing that it would just suddenly start taking effect ~3 billion years ago for no apparent reason. Are there any theories as to what could have possibly brought dark energy into existence at that point in time?

Thanks for the lengthy reply by the way, all fascinating stuff!

Well actually, you're right that it has always been around. But its influence relative to the matter and radiation density of the universe has been changing over time. If you can bear a bit of maths, the following connects all the important cosmological parameters (it's a Friedmann equation based on General Relativity and the Friedmann–Lemaître–Robertson–Walker metric for a homogeneous isotropic expanding universe :eek:, but that's not important right now ):



The above relates the "Hubble parameter" (the rate of change of the cosmic scale factor, effectively the rate of expansion) to the matter and radiation densities , the dark energy (), and the curvature k.

In the nice simple universe that Einstein supposed, where the dark energy (he used a different name for it) was zero and space was flat (i.e. k=0) then the expansion rate just depended on the matter and radiation density. This was called the Einstein-deSitter model. The expansion rate was high in the early universe when the density was high, and lower as the density falls off with the expansion of the universe. Whether or not the universe expanded forever would be solely determined by whether the matter density was above or below some critical density.

However, a non-zero dark energy component complicates things considerably, and leads to several possible different universes depending on the ratios of the matter, energy and dark energy densities and the curvature. It can give us a universe that expands rapidly, then slows, but then takes off again.

One way to picture this is that in the extremely early universe, radiation dominates and produces an expansion pressure. As the universe expands, it's volume goes up with the cube of the radius. So the matter density falls correspondingly. However, the radiation density is also affected by the cosmological red shift and thus falls with the fourth power of the radius ( - as well as photons being spread more thinly, each photon's energy is reduced by being redshifted). So, instead of being radiation dominated, the universe enters a matter-dominated phase where gravity is slowing down the expansion. But the matter density continues to fall, and if we have a dark energy which is proportional to the overall volume of space, it starts to take over as the universe gets bigger, so things begin to accelerate. We don't know what such a form of energy would be, but there is speculation about a "vacuum energy" produced by space itself. (Don't ask me, that's as much as I know ).

Zubeneschamali

ps200306 wrote: »

there is speculation about a "vacuum energy" produced by space itself.

Harnessing Vacuum Energy would be all very well, but I don't use the vacuum cleaner as much as I should, I'd prefer to harness Home Heating Energy (aka Diesel Energy)

Of course, we must avoid Red/Green Diesel Energy for cars.

Carawaystick

syngindub wrote: »

Is it true we can see the remnant's of the Big Bang from Static on tv?

sort of.
Two lads Penzias & Wilson were testing a radio back in the '60's and were still getting more noise than their calculations reckoned (noise ~= static on a tv)

turned out the excess noise was the cosmic background radiation
and the two lads got a nice trip to Stockholm for a Nobel prize

Most of the static (or hiss on a radio) is noise generated by the electronics in the tv/radio itself, followed by noise generated by other electrical things in your house.

Capt'n Midnight

Zubeneschamali wrote: »

Harnessing Vacuum Energy would be all very well, but I don't use the vacuum cleaner as much as I should,

there isn't much energy in a vacuum and Maxwells deamon doesn't work when the gradient builds up so meh.

we have lots of solar though and a big enough rectanna matrix would enable you harvest the cosmic background

ps200306

Zubeneschamali wrote: »

Harnessing Vacuum Energy would be all very well, but I don't use the vacuum cleaner as much as I should...

So if you hear a big sucking sound, you'll know it's the end of the universe, and not your house being cleaned.

Actually, I believe it's going to be a big ripping sound ... there's one theory that the runaway expansion due to dark energy will first accelerate all the galaxies out of our observable universe, but eventually the galaxy will start coming apart too, and then stars and planets, and eventually the individual atoms as even the intra-atomic forces are finally overwhelmed. It's called, appropriately, the Big Rip:

"About 60 million years before the end, gravity would be too weak to hold the Milky Way and other individual galaxies together. Approximately three months before the end, the solar system... would be gravitationally unbound. In the last minutes, stars and planets would be torn apart, and an instant before the end, atoms would be destroyed."

ps200306

Capt'n Midnight wrote: »

...a big enough rectanna matrix would enable you harvest the cosmic background

REALLY?
I've never heard of such a thing, and know less than nothing about it, but I'm thinking -- on thermodynamic principles alone -- harvesting energy from something at three degrees above absolute zero would need a cold sink that's even colder than that, which is next to impossible to come by for free in a universe which is all at the same three degree temperature. I know we're talking about a black body temperature of EM radiation here, but still .... !

TheMilkyPirate

This thread has melted my head.

Zubeneschamali

ps200306 wrote: »

REALLY? ... !

No, that was a joke.

But the bit about Hoovers...

ps200306

TheMilkyPirate wrote: »

This thread has melted my head.

Maybe we could harvest energy from that too

Zubeneschamali wrote: »

No, that was a joke.

But the bit about Hoovers...

Squaredude

Ah sure the universe is just hologram anyway

bombs away

Well as far as I concerned it's just one of many multiverses so as soon as this one goes there'll be another to take it's place


Wont be around to see it though

Taylor365

Great read!

animaal

ps200306 wrote: »

"About 60 million years before the end, gravity would be too weak to hold the Milky Way and other individual galaxies together. Approximately three months before the end, the solar system... would be gravitationally unbound. In the last minutes, stars and planets would be torn apart, and an instant before the end, atoms would be destroyed."

Noooo, we were just a few days from exiting the bailout and waving goodbye to the troika for good!

ps200306

animaal wrote: »

Noooo, we were just a few days from exiting the bailout and waving goodbye to the troika for good!

Look at this way. Probably the only way we're going to get out of BAILOUT-II is to be ripped from it by an irresistible force that tears the universe asunder. :pac: