Richard Feynman used to say that if you can't explain something in an undergraduate lecture then you don't truly understand it. Perhaps these PhD students didn't truly understand it yet...?
Astronomy / Space Is the universe expanding faster than the speed of light?
Richard Feynman used to say that if you can't explain something in an undergraduate lecture then you don't truly understand it. Perhaps these PhD students didn't truly understand it yet...?
He was a true genius IMHO: his book on chromodynamics is wonderful.
Well, I'm just an amateur so don't feel it's fair to judge those much wiser than me. But I think you're right: having spoken to someone who didn't complete his Physics degree because he said it was just applied maths with no "blue sky" thinking at all.
In my field, I know it's easy to get bogged down in the day to day minutiae without looking at the big picture.
I gues that's what made Einstein so amazing: he was willing to overthrow the entire way of looking at the world because of a single fact that didn't fit, produced elegant equations, was convinced they were right because of their beauty, and gave simple gedankenexperiments (like Feynman) that we all felt we could understand.
Though personally I still struggle with Spacetime :S
[geek_mode]
Galaxies at opposite ends of the Universe are not moving away from each other: The galaxies appear stationary to observers within them. The space between them is expanding.
Just as it is a property of (some types of) matter that it has mass, and a property of mass that it warps spacetime, it is a property of space that it expands at a constant (though very small) rate.
e.g. If you have a sphere of empty space 1 light year in diameter (I don't have the actual figures to hand, but for example...) it expands at a rate of 1 cm/sec... and a second later it is 1 cm larger, so it is now expanding at 1.00000000000000003 cm/sec, and a second later... It's a bit like compound interest. You get the idea.
Multiply this by billions of light years over billions of years, and this is why the rate of expansion appears to be accelerating.
As for inflation, think of it like this: (to make the numbers and speeds more easy to imagine let's say the "speed of sound" rather than the "speed of light" for this example):
One theory is that the Big Bang was actually a collision between 2 four-dimensional "branes" in a multi-dimensional Multiverse.
Imagine you are at the North pole, and a 2 dimensional plane (actually the surface of a brane) is moving "southwards" at the speed of sound, just about to reach the surface of the Earth. Look around you: the surface of the Earth appears flat, though you know it is very slightly curved.
When the plane/brane reaches the surface, the area of the surface it intersects with appears to expand faster than the speed of sound, and as it moves further south - through the globe, the rate of expansion appears to slow down. But the plane/brane's speed is constant through space.
Of course, this is where the analogy breaks down, because the Earth is roughly spherical, so when the plane/brane reaches the equator, the expansion will appear to reverse, and we know that this is not the case: Inflation started REALLY fast, then slowed down dramatically, and is now slowly accelerating.
Therefore the "shape" of the expansion of the Universe must be a bit like a cone, with a sort of at the top, and flaring out at the bottom.
Yes folks, from the outside, our four-dimensional Universe looks like a mind-bogglingly, enormously huge sex toy.
[/geek_mode]
Galaxies at opposite ends of the Universe are not moving away from each other: The galaxies appear stationary to observers within them. The space between them is expanding.
Just as it is a property of (some types of) matter that it has mass, and a property of mass that it warps spacetime, it is a property of space that it expands at a constant (though very small) rate.
e.g. If you have a sphere of empty space 1 light year in diameter (I don't have the actual figures to hand, but for example...) it expands at a rate of 1 cm/sec... and a second later it is 1 cm larger, so it is now expanding at 1.00000000000000003 cm/sec, and a second later... It's a bit like compound interest. You get the idea.
Multiply this by billions of light years over billions of years, and this is why the rate of expansion appears to be accelerating.
As for inflation, think of it like this: (to make the numbers and speeds more easy to imagine let's say the "speed of sound" rather than the "speed of light" for this example):
One theory is that the Big Bang was actually a collision between 2 four-dimensional "branes" in a multi-dimensional Multiverse.
Imagine you are at the North pole, and a 2 dimensional plane (actually the surface of a brane) is moving "southwards" at the speed of sound, just about to reach the surface of the Earth. Look around you: the surface of the Earth appears flat, though you know it is very slightly curved.
When the plane/brane reaches the surface, the area of the surface it intersects with appears to expand faster than the speed of sound, and as it moves further south - through the globe, the rate of expansion appears to slow down. But the plane/brane's speed is constant through space.
Of course, this is where the analogy breaks down, because the Earth is roughly spherical, so when the plane/brane reaches the equator, the expansion will appear to reverse, and we know that this is not the case: Inflation started REALLY fast, then slowed down dramatically, and is now slowly accelerating.
Therefore the "shape" of the expansion of the Universe must be a bit like a cone, with a sort of at the top, and flaring out at the bottom.
Yes folks, from the outside, our four-dimensional Universe looks like a mind-bogglingly, enormously huge sex toy.
[/geek_mode]
Accelerating expansion means what the word says. Today it has one rate, next year the rate will be larger.
Can get you some numbers or graphs if you like..
But accelerating expansion (rather than there being more & more space that is expanding) would imply that the speed of light is changing over time, and there's no evidence for that.
Yes please on the graphs & numbers, but nothing too complicated - I'm a fan of science but I don't do maffs. :S
Okay, sorry for using expansion in a confusing way.
The universe is expanding.
Reason is, there is more and more space.
In other words, space is not stretching, more space gets 'born' all the time.
So if you have a meter, later that distance will be longer. The definition of a meter will however not follow the expansion. With time there will just be more meters, if you get what I mean.
About light, light travelling through growing space, will get affected, but not its speed. Its wavelength gets longer the longer it travels. So light from a distant star will be more red when it reaches us. This effect is red-shift, there is also another cause for red-shift called relativistic red-shift, which is caused by time dilation.
So light from a galaxy 1mil light-year away, will take more than 1mil years to reach us, since its way here gets longer all the time.
Will get you a nice graph.
Okay, sorry for using expansion in a confusing way.
The universe is expanding.
Reason is, there is more and more space.
In other words, space is not stretching, more space gets 'born' all the time.
So if you have a meter, later that distance will be longer. The definition of a meter will however not follow the expansion. With time there will just be more meters, if you get what I mean.
About light, light travelling through growing space, will get affected, but not its speed. Its wavelength gets longer the longer it travels. So light from a distant star will be more red when it reaches us. This effect is red-shift, there is also another cause for red-shift called relativistic red-shift, which is caused by time dilation.
So light from a galaxy 1mil light-year away, will take more than 1mil years to reach us, since its way here gets longer all the time.
Will get you a nice graph.
I thought that's what I said.:S
Anyway, It's nice to agree with you.
Edit: Apart from relativistic red-shift, which I forgot about. I believe that's when light passes by a black hole (for instance) and gets briefly slowed in the gravity well due to time-dilation rather than due to actually slowing down, as in passing through a medium.
I think.
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Sort of, apart from the changing light speed. There is no evidence it has changed over time.
Instead of showing you the expansion rate, I think it's more intuitive to look at the scale factor.
Its a factor for how a length changes over time:
So to use it, take two object today, and multiply their distance with the scale factor from the time you are interested in to see how far away from each other they were back then (or in future). Only works for objects which is so far apart that they do not gravitationally influence each other much (milkyway-andromeda wont work for example).
And here it is again, with the Factor shown in logarithmic scale, to better show how it curves:
The expansion rate now, put as Hubble's Constant is sorta:
70 +/- 2.2 km/sec/MegaParsec
Its a factor for how a length changes over time:
So to use it, take two object today, and multiply their distance with the scale factor from the time you are interested in to see how far away from each other they were back then (or in future). Only works for objects which is so far apart that they do not gravitationally influence each other much (milkyway-andromeda wont work for example).
And here it is again, with the Factor shown in logarithmic scale, to better show how it curves:
The expansion rate now, put as Hubble's Constant is sorta:
70 +/- 2.2 km/sec/MegaParsec
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That's what I said!!!!!!!!!!!!!1!!!!!!!!!!!!!!eleventyone!
Just reread what you wrote. I did misunderstand it.
Very sorry. You are right. *goes hide under under a bush somewhere*
Still hope you enjoy the graphs, even though they are posted by a moron.
One way expansion was explained to me which I found rather helpful is with the following analogy:
Imagine the entire observable universe is painted on the surface of a (hopefully infinitely expandable) "balloon". The beginning of time occurred when the "balloon" started inflating. From that point forward everything on the surface started appearing to move away from each other (decreasing pressure and heat in the process), in actual fact they were stationary, it was just the "balloon" getting bigger.
Now everything on the surface of this "balloon" can still slide around this surface and affect each other though things like gravity and more localised forces, but of course the speed they can slide around this surface is limited to the speed of light.
So the apparent movement due to expansion can be considered the rest state. There is no actual movement along the surface of the "balloon" happening due to expansion, it is the universe itself getting bigger.
This analogy can be taken further to explain other concepts most notably a 4th spatial dimension, the big rip, dark matter and many worlds theories, but that isn't really the subject at hand and it gets a little bit more far fetched at that point.
Imagine the entire observable universe is painted on the surface of a (hopefully infinitely expandable) "balloon". The beginning of time occurred when the "balloon" started inflating. From that point forward everything on the surface started appearing to move away from each other (decreasing pressure and heat in the process), in actual fact they were stationary, it was just the "balloon" getting bigger.
Now everything on the surface of this "balloon" can still slide around this surface and affect each other though things like gravity and more localised forces, but of course the speed they can slide around this surface is limited to the speed of light.
So the apparent movement due to expansion can be considered the rest state. There is no actual movement along the surface of the "balloon" happening due to expansion, it is the universe itself getting bigger.
This analogy can be taken further to explain other concepts most notably a 4th spatial dimension, the big rip, dark matter and many worlds theories, but that isn't really the subject at hand and it gets a little bit more far fetched at that point.
Instead of showing you the expansion rate, I think it's more intuitive to look at the scale factor.
Its a factor for how a length changes over time:
The expansion rate now, put as Hubble's Constant is sorta:
70 +/- 2.2 km/sec/MegaParsec
That reminds me of the Kickstarter graph! there must be a Univeral Truth out there somwhere...
Getting back to Feynman, just caught one of his lectures on youtube where he basically says "just add the arrows, don't try to work out how they work". But that feels like giving up to me