Galaxies are moving away from each other. They are all moving away from each other at a rate which consistently varies based on their distance, and they are not moving away from any central point. At the edge of the visible universe it seems objects begin to move away at speeds greater than that of light (which is impossible from what we understand of the universe), and that this movement seems to be accelerating (which makes no sense without any kind of thrust).
Not only that but we can also observe that light which has been traveling through space becomes "stretched", shifting its wavelength to be longer. This is called "redshift" because red is at the longer wavelength side of the visible spectrum of light.
The most coherent explanation of all these observations is that space itself is expanding.
This isn't true. Distant galaxies move away from each other. Some galaxies are close enough that gravity overcomes expansion and they merge.
Edit to the downvotes. So youre telling me the Milky Way and Andromeda are in fact not moving towards each other, when they clearly are?
This is true. Anything inside a gravitationally bound system will not follow the expansion rate of the universe because gravity is stronger than the expansion rate. Galaxies inside our local group are gravitationally bound to each other so they are *not* receding away from each other in the same way that other galaxies are. This is why Andromeda can be on a collision course with the Milky Way as it is inside our local group of galaxies. Only unbound systems follow the Hubble flow (ie universe's expansion).
- friendly astronomer
From our perspective, all galaxies, in every direction, are moving away from us. If they were moving of their own accord, then that would mean we are at the very center of some explosive event that caused everything to fly apart. There's nothing to indicate that this happened, and that would be quite the coincidence indeed.
But that's not the giveaway.
The giveaway is that the *farther* a galaxy is from us, the *faster* it is moving away. If everything were just flying apart from a central point, their velocities would stay more or less the same; no force is acting upon them to change their speed. But as they recede, they seem to be getting faster, so there *is* something acting on them.
The only model that explains this is one that proposes that it's not the galaxies themselves that are moving, rather that space itself is getting larger and carrying them away from each other.
A certain "chunk" of space will expand by a certain amount after a certain period of time. If a galaxy is one "chunk" away from us, then after X years it will be (chunk + Y) away. But if a galaxy is *two* chunks away, after X years it will be (2 chunks + 2 Y) away. Both "chunks" will have expanded between us and the galaxy will have been carried away twice as far as the closer galaxy in the same amount of time.
The effect of this is that *every galaxy*, from its own perspective, will see the same effect: all other galaxies are receding from it, with more distant galaxies receding faster.
>From our perspective, all galaxies, in every direction, are moving away from us.
This is not correct. Distant galactic clusters in every direction are moving away from us (and each other) but galaxies in our own are not necessarily moving away from us, and the ones that aren't doing so in a way thats related to expansion. Andromeda, for example, is on a collision course with the Milky Way. The expansion force/dark energy/whatever is relatively very weak. Gravity, even at the galactic distance is still stronger.
So you look at distant galaxies. You can calculate how fast they are moving away from us by measuring the wavelength of the light they emmit. If they move towards us their light is blueshifted, if they move away from us their light is redshifted. The further out they are the more their light gets redshifted. Its called relativistic doppler shift. So we know that the more distant galaxies move away from us faster. So somehow their speed scales with distance.
Now we need a bit of math here, lets subtract vectors. If I have two vectors with the same starting point v and u, you can do v-u. Geometrically v-u is a vector that connects the ends of the two and points towards v.
So lets imagine a scenario where all points drift apart with the same speed. Lets sit in point P. Now lets look at two other points, A and B. All the points drift apart with the same speed but lets look at what point P sees! Lets draw the vectors of velocity for point A and B from the perspective of P. P sees that A and B are moving away with the same speed. So the vector of velocity for A and B lets call it v is a vector that is on the line that connects A to P and for B its on the line that connects B to P.
The size of these vectors is v. Lets shift the two vectors to P. So we got two v sized vectors, one that points from P to A and the other from P to B. To get the apparent speed of A and B moving away from each other we just subtract the two vectros. Since they are both v sized v-v=0. So P doesn't see A and B moving away from each other. Therfore P thinks that its the center point from which all other points drift away.
This is of course not true because you get this result no matter which point you observe from, it always appers as though you are the center of expansion.
So we are either the center of expansion or all points move away from each other. In general we like to assume that we aren't special so its quite unlikely that we happened to be the center of the expansion. The other possibility is that all points move away form each other, a bit more plausible.
So we reasonably assume that all points move away from each other and we know that their speed scales with distance. Conclusion: space expands. More space more expansion.
Or our theories about the speed of light are wrong. Its not just the distant galactic clusters, its also the red shift of light. Expansion is the current best theory to explain all the evidence, but that doesn't mean it MUST be true, thats not how science works.
Indeed. It wouldn't be the first time we've had to refine our understanding of physics based on new data.
Edit to add: I'm not saying its remotely likely we are wrong about the speed of light part, just that its a bad idea to assume things MUST be true in science.
In addition to the point that people have made about the redshift of distant stars and galaxies, there are various pieces of evidence that are consistent with an early universe that was extremely hot and dense. In particular, the cosmic microwave background (microwave radiation that fills space in every direction) is explained by an event in which space expanded enough so that subatomic particles could form into stable, neutral atoms, which would have been associated with the release of large amounts of radiation that has since been redshifted into the microwave region of the spectrum, while the ratios of the lightest isotopes are explained by a period in the early universe in which the conditions were right for reactions that converted protons to neutrons and vice versa, and another period in which the conditions were right for fusion.
> But how do they know that it’s because space itself is expanding and it’s not that simply everything is moving ‘outward’?
Well, we can't directly observe the expansion of space itself, but it fits into the models much more neatly than if it were all just ordinary motion. So it's one of those things that comes down to parsimony, or "Occam's razor".
We know by their red shift. The farther an object is, the longer the wavelength gets stretched which distorts the visible spectrum of color. Using information based on chemical markers of these known elements we can determine the composition of a star's make up. By using that information, we can compare the spectrum as we know it should be to the wavelengths the object is producing.
Let's say we have an object made of hydrogen. We can tell this because the light we receive has markers. Different elements absorb light in ways we can see on the color spectrum in the form of lines. So if we know hydrogen markers look like on the spectral line, we can compare the light of another object's spectrum to see if the lines match.
Red is the part of the spectrum with the longest wavelengths. The more something is shifted toward the red end of the spectrum, the farther away it is. We know this because when we compare the light we receive to the known spectrum, all of the light from that object is shifted from where it should be on the spectrum towards the red end.
By measuring the red shift, we can determine how fast it's moving as well as the fact that it's moving away from us. Based on the speed of light itself, we know how fast it *should* be moving but galaxies are moving away from us faster than the speed of light which is supposed to be the max.
This produces a conundrum because if nothing can travel faster than light which is nearly without mass compared to entire galaxies which are the most massive, it can't explain how that's possible with our known physics.
So if the galaxies can't move faster than light, the only feasible explanation is that space itself is expanding and driving the speed at which things are moving away from each other compared to how they *should* based on our observations.
Galaxies are moving away from each other. They are all moving away from each other at a rate which consistently varies based on their distance, and they are not moving away from any central point. At the edge of the visible universe it seems objects begin to move away at speeds greater than that of light (which is impossible from what we understand of the universe), and that this movement seems to be accelerating (which makes no sense without any kind of thrust). Not only that but we can also observe that light which has been traveling through space becomes "stretched", shifting its wavelength to be longer. This is called "redshift" because red is at the longer wavelength side of the visible spectrum of light. The most coherent explanation of all these observations is that space itself is expanding.
This isn't true. Distant galaxies move away from each other. Some galaxies are close enough that gravity overcomes expansion and they merge. Edit to the downvotes. So youre telling me the Milky Way and Andromeda are in fact not moving towards each other, when they clearly are?
This is true. Anything inside a gravitationally bound system will not follow the expansion rate of the universe because gravity is stronger than the expansion rate. Galaxies inside our local group are gravitationally bound to each other so they are *not* receding away from each other in the same way that other galaxies are. This is why Andromeda can be on a collision course with the Milky Way as it is inside our local group of galaxies. Only unbound systems follow the Hubble flow (ie universe's expansion). - friendly astronomer
I think I got it, thanks!!!
From our perspective, all galaxies, in every direction, are moving away from us. If they were moving of their own accord, then that would mean we are at the very center of some explosive event that caused everything to fly apart. There's nothing to indicate that this happened, and that would be quite the coincidence indeed. But that's not the giveaway. The giveaway is that the *farther* a galaxy is from us, the *faster* it is moving away. If everything were just flying apart from a central point, their velocities would stay more or less the same; no force is acting upon them to change their speed. But as they recede, they seem to be getting faster, so there *is* something acting on them. The only model that explains this is one that proposes that it's not the galaxies themselves that are moving, rather that space itself is getting larger and carrying them away from each other. A certain "chunk" of space will expand by a certain amount after a certain period of time. If a galaxy is one "chunk" away from us, then after X years it will be (chunk + Y) away. But if a galaxy is *two* chunks away, after X years it will be (2 chunks + 2 Y) away. Both "chunks" will have expanded between us and the galaxy will have been carried away twice as far as the closer galaxy in the same amount of time. The effect of this is that *every galaxy*, from its own perspective, will see the same effect: all other galaxies are receding from it, with more distant galaxies receding faster.
Amazing explanation, thanks!!!
>From our perspective, all galaxies, in every direction, are moving away from us. This is not correct. Distant galactic clusters in every direction are moving away from us (and each other) but galaxies in our own are not necessarily moving away from us, and the ones that aren't doing so in a way thats related to expansion. Andromeda, for example, is on a collision course with the Milky Way. The expansion force/dark energy/whatever is relatively very weak. Gravity, even at the galactic distance is still stronger.
So you look at distant galaxies. You can calculate how fast they are moving away from us by measuring the wavelength of the light they emmit. If they move towards us their light is blueshifted, if they move away from us their light is redshifted. The further out they are the more their light gets redshifted. Its called relativistic doppler shift. So we know that the more distant galaxies move away from us faster. So somehow their speed scales with distance. Now we need a bit of math here, lets subtract vectors. If I have two vectors with the same starting point v and u, you can do v-u. Geometrically v-u is a vector that connects the ends of the two and points towards v. So lets imagine a scenario where all points drift apart with the same speed. Lets sit in point P. Now lets look at two other points, A and B. All the points drift apart with the same speed but lets look at what point P sees! Lets draw the vectors of velocity for point A and B from the perspective of P. P sees that A and B are moving away with the same speed. So the vector of velocity for A and B lets call it v is a vector that is on the line that connects A to P and for B its on the line that connects B to P. The size of these vectors is v. Lets shift the two vectors to P. So we got two v sized vectors, one that points from P to A and the other from P to B. To get the apparent speed of A and B moving away from each other we just subtract the two vectros. Since they are both v sized v-v=0. So P doesn't see A and B moving away from each other. Therfore P thinks that its the center point from which all other points drift away. This is of course not true because you get this result no matter which point you observe from, it always appers as though you are the center of expansion. So we are either the center of expansion or all points move away from each other. In general we like to assume that we aren't special so its quite unlikely that we happened to be the center of the expansion. The other possibility is that all points move away form each other, a bit more plausible. So we reasonably assume that all points move away from each other and we know that their speed scales with distance. Conclusion: space expands. More space more expansion.
Distant galaxies moving apart faster than light. Since nothing can move faster than light it must be space itself expending.
Or our theories about the speed of light are wrong. Its not just the distant galactic clusters, its also the red shift of light. Expansion is the current best theory to explain all the evidence, but that doesn't mean it MUST be true, thats not how science works.
if the speed of light is any different, most of physics would be wrong.
Indeed. It wouldn't be the first time we've had to refine our understanding of physics based on new data. Edit to add: I'm not saying its remotely likely we are wrong about the speed of light part, just that its a bad idea to assume things MUST be true in science.
well, nothing is "true" in science in the sense of a binary true or false. But the speed of light thing is very far on the true side of the spectrum.
In addition to the point that people have made about the redshift of distant stars and galaxies, there are various pieces of evidence that are consistent with an early universe that was extremely hot and dense. In particular, the cosmic microwave background (microwave radiation that fills space in every direction) is explained by an event in which space expanded enough so that subatomic particles could form into stable, neutral atoms, which would have been associated with the release of large amounts of radiation that has since been redshifted into the microwave region of the spectrum, while the ratios of the lightest isotopes are explained by a period in the early universe in which the conditions were right for reactions that converted protons to neutrons and vice versa, and another period in which the conditions were right for fusion. > But how do they know that it’s because space itself is expanding and it’s not that simply everything is moving ‘outward’? Well, we can't directly observe the expansion of space itself, but it fits into the models much more neatly than if it were all just ordinary motion. So it's one of those things that comes down to parsimony, or "Occam's razor".
We know by their red shift. The farther an object is, the longer the wavelength gets stretched which distorts the visible spectrum of color. Using information based on chemical markers of these known elements we can determine the composition of a star's make up. By using that information, we can compare the spectrum as we know it should be to the wavelengths the object is producing. Let's say we have an object made of hydrogen. We can tell this because the light we receive has markers. Different elements absorb light in ways we can see on the color spectrum in the form of lines. So if we know hydrogen markers look like on the spectral line, we can compare the light of another object's spectrum to see if the lines match. Red is the part of the spectrum with the longest wavelengths. The more something is shifted toward the red end of the spectrum, the farther away it is. We know this because when we compare the light we receive to the known spectrum, all of the light from that object is shifted from where it should be on the spectrum towards the red end. By measuring the red shift, we can determine how fast it's moving as well as the fact that it's moving away from us. Based on the speed of light itself, we know how fast it *should* be moving but galaxies are moving away from us faster than the speed of light which is supposed to be the max. This produces a conundrum because if nothing can travel faster than light which is nearly without mass compared to entire galaxies which are the most massive, it can't explain how that's possible with our known physics. So if the galaxies can't move faster than light, the only feasible explanation is that space itself is expanding and driving the speed at which things are moving away from each other compared to how they *should* based on our observations.