[Physics] If the universe was expanding at the speed of light... [on a treadmill]

J

Jagonath

Veteran X
I've got a phsyics/astronomy question I don't understand.

According to most accounts, the universe is 13.7 billion years old.

Light travels, er, at the speed of light. Thus, the radius of the observable universe is 13.7 billion light years, and the diameter approximately 27 billion light years. Duh, right?

Wrong.

According to wikipedia the universe "may be thought of as a perfect sphere with the earth at its center and a diameter of about 93 billion light years."

Universe - Wikipedia, the free encyclopedia

I'll admit it. I'm dumb. I know the technical explanation by these "physicists" aka students-of-magic, but it was like trying to read War and Peace in morse-code. Backwards.

Can someone explain this shit using concepts and words that actual people can understand?
 
What happens if the people at the edge of the universe switch on their lights - does the universe expand?
 
The answer is we don't know shit and we're making it up as we go along to make it seem like our scientists are doing something.
 
This reasoning might make sense if we lived in the flat spacetime of special relativity, but in the real universe spacetime (not space) is highly curved at cosmological scales, and light does not move rectilinearly. Distances obtained as the speed of light times a cosmological time interval have no direct physical significance.
 
Animo said:
This reasoning might make sense if we lived in the flat spacetime of special relativity, but in the real universe spacetime (not space) is highly curved at cosmological scales, and light does not move rectilinearly. Distances obtained as the speed of light times a cosmological time interval have no direct physical significance.
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Nice cut and paste from Observable universe - Wikipedia, the free encyclopedia

:rolleyes:

What the fuck is a cosmological time interval? Why do they use different measurements to measure the speed of light and the age of the universe? What is the point of measuring this stuff at all if the measurements can essentially mean anything?

What the hell is does rectilinearly mean? If you ask me, I think you pulled that rectilinear explanation right out of your ass!
 
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I don't know shit about this, but I think the important thing is that 'it may be thought of' rather that 'it is'
 
Jagonath said:
According to most accounts, the universe is 13.7 billion years old.

Light travels, er, at the speed of light. Thus, the radius of the observable universe is 13.7 billion light years, and the diameter approximately 27 billion light years. Duh, right?
Click to expand...

A couple things.

Cosmic inflation is the theory that when the universe was very young, it expanded exponentially, much faster than the speed of light. So what we can ever see is a small part of the universe, much/most/who the fuck knows how much of it went flinging off faster than light, never to be seen again.

Also Datrk Energy, supernova surveys appear to reveal that the expansion of the universe is accelerating.

Also keep in mind that distance is a meaninglessly vague term when talking about cosmology, the proper term is "lookback time." If we observe a galaxy to be a billion light years away, we are really looking at how long the light has traveled, not the distance. The distance to what? When it emitted the light, it was much closer, and now a billion years later, it's much farther. Time of travel is the only really meaningful description of distance at those scales.
 
Jagonath said:
I've got a phsyics/astronomy question I don't understand.

According to most accounts, the universe is 13.7 billion years old.

Light travels, er, at the speed of light. Thus, the radius of the observable universe is 13.7 billion light years, and the diameter approximately 27 billion light years. Duh, right?

Wrong.

According to wikipedia the universe "may be thought of as a perfect sphere with the earth at its center and a diameter of about 93 billion light years."

Universe - Wikipedia, the free encyclopedia

I'll admit it. I'm dumb. I know the technical explanation by these "physicists" aka students-of-magic, but it was like trying to read War and Peace in morse-code. Backwards.

Can someone explain this shit using concepts and words that actual people can understand?
Click to expand...

the visible universe may be thought of as a sphere with the Earth in the center.

Think about it like this, it takes a long ass time for light to reach us from way the fuck away. The furthest we can see in basically all directions is about the same period of time and distance for light to travel to us.

Effectively what we have is a snapshot of the universe in that we are talking about light traveling billions of years and us only having about 50 years of data at best from the furthest reaches.
 
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Automatic Jack said:
A couple things.

Cosmic inflation is the theory that when the universe was very young, it expanded exponentially, much faster than the speed of light. So what we can ever see is a small part of the universe, much/most/who the fuck knows how much of it went flinging off faster than light, never to be seen again.

Also Datrk Energy, supernova surveys appear to reveal that the expansion of the universe is accelerating.

Also keep in mind that distance is a meaninglessly vague term when talking about cosmology, the proper term is "lookback time." If we observe a galaxy to be a billion light years away, we are really looking at how long the light has traveled, not the distance. The distance to what? When it emitted the light, it was much closer, and now a billion years later, it's much farther. Time of travel is the only really meaningful description of distance at those scales.
Click to expand...

Nothing, individually, can go faster than the speed of light.

But, two objects moving away from each other, going .75 times the speed of light, can collectively move away from each other at a rate that would cause light to take far longer to get to the other object.



As to what gravity does to space time in MASSIVE scales (think the whole universe) it actually deforms all of space. Distances are warped by the geometry of our entire universe. You can almost think of the universe as a massive sphere:

Im sure we've all seen those examples of gravity wells that use a sheet or membrane stretched across, with a bowling ball in the middle. This is a nice illistration of how space time is warped by a massive object.

Now, imagine if you mapped ALL of space time. Instead of a flat section of membrane, the membrane would by a sphere, a sphere that is constanlty expanding outward at every point of its surface.

This is how the actual length of the universe can be much larger than what we can see: if every point of the shereical 'membrane' is expanding, then distances would grow at a much larger rate than what light can travel.

So, the light you see now was shined a fuck all of a long time ago. In that time that that light was traveling, the spacetime between the object and us has expanded soo much that now new light being emitted can never get here.



Side note - Another example: Think about how all of space time is expanding, even in the distance from one wall to another in the room you're in.

Light can easily travel from one wall to the other, even though the space time between the walls is constantly expanding, just at a tiny fraction. Now imagine if those walls were 2 billion light years apart with the same constant expansion at every piont. All those 'points' expanding would collectively push the 'real' distance between those walls at a massive rate, a rate that's not noticeable in the scale of just a room.
 
haniblecter said:
This is how the actual length of the universe can be much larger than what we can see: if every point of the shereical 'membrane' is expanding, then distances would grow at a much larger rate than what light can travel.
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Interesting. Is the "stretching of the membrane" also equivalent to time speeding up?
 
It's also seems difficult to make much sense out of speed in a setting where someone is driving away from you at 30 mph, yet at the end of one hour they are 110 miles away from you.
 
Jagonath said:
Nice cut and paste from Observable universe - Wikipedia, the free encyclopedia

:rolleyes:

What the .... is a cosmological time interval? Why do they use different measurements to measure the speed of light and the age of the universe? What is the point of measuring this stuff at all if the measurements can essentially mean anything?

What the hell is does rectilinearly mean? If you ask me, I think you pulled that rectilinear explanation right out of your ...!
Click to expand...

You know you can look this up yourself, that is why I simply used a cut and paste.

Also, I would recommend against using profanity in order to express yourself, it is a sign of poor intelligence.
 
godswill said:
or would that be time slowing down..
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It would be speeding up. If you think of it as today's light years being smaller (more of them fit between things than they used to), one way to think of that would be that years are not as long as they used to be.
 
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