4 ms·

> that galaxies are moving away moving away from what? The reason inflation works is that it doesn't require a privileged frame of reference.

scotty79 3y ago
From each other. Bigbang that has a center is perfectly consistent with Hubble law. Imagine that the spacetime is flat, there's no gravity and clumps of matter (galaxies) initially had random velocities. The faster the galaxy was, the further is went till now. What is a bit surprising, is that if you do the math in such universe and compare distances from any galaxy to others and their relative velocities you'll get Hubble law exactly. For every galaxy the distance to any other galaxy is directly proportional to relative velocity between them and they appear to be moving directly away from each other. So even though this big bang had epicenter you can't really tell that it had it or where it was if you can't see the edge. And if our observable universe is small when compared with the size of the entire universe we have a low chance of being near the edge. Try it. You can do it on paper or do a little computer simulation and measure. I did it few years ago. I was quite surprised with the results. Especially with the direction of the relative velocity vectors. They always face directly away from one galaxy to any other.
- gpderetta 3y ago
  > From each other. Bigbang that has a center is perfectly consistent with Hubble law. So we see all distant galaxies moving radially away us. Are you saying that the big bang happened to be centered, of all the universe, around the Milky Way?
  - scotty79 3y ago
    No. I'm saying that if big bang imparted random velocities on galaxies that were all initially in the same spot (epicenter) then tofay in the reference frame of any galaxy (also Milky Way) it will look like every other galaxy is moving away from it radially with the velocity proportional to the current distance from it. No one will be able to tell where the actual epicenter was if they are too far from the edge to see it. This holds for the universe without gravity, with flat spacetime, Newtonian physics, with three, two or one spatial dimension. Seriously, try to simulate such toy universe. Or just sit down with a piece of paper and consider variant with one spatial dimension. It's primary school math and physics. You'll get Hubble law.
    - simonh 3y ago
      You’re right about the distribution of the velocities of galaxies. The problem would be the distribution of the density of galaxies. Even in an instantaneous snapshot of the state of the universe now, there would be more galaxies in the direction of the epicentre from us, say assuming we had average velocity. To see an even distribution of actual galaxies (not their velocities) in the sky we’d need to have zero initial velocity. But also light would still take time to travel. Our view of galaxies 13bn light years away would still be of them as they were 13bn years ago in the very early universe. Again unless we happened to have zero initial velocity, we’d see all the distant galaxies clustered together in one spot in the sky.
      - scotty79 3y ago
        We don't know of the distribution of galaxies in our universe is exactly equal. And the exact amount of any anisotropy is highly dependant on how small fraction of the whole universe is observable by us and how far are we from epicenter and the edge. 13bn light-years worth of galaxies might be too small sample to notice any asymetry with our current equipment. What if that's just one billionth of all the matter thrown apart by the big bang? Universe did not necessarily start from a point, just from a very energetic state where the all matter we can see now was fairly close together.
        simonh 3y ago
        We can see the relative motion of galaxies up to the edge of the observable universe from their red shift. So we know they were all 'in one place' 13 bn years ago. It also means we know what the distribution of initial vectors must have been in this scenario for these galaxies. So either the conventional view is correct and it's space that has expanded moving the galaxies further apart. This is consistent with a potentially infinite universe. Alternatively all the galaxies we can see were clustered together in a tight lump 13bn years ago and flew apart. If the latter is the case the origin point would still be visible to us. As long as our initial random velocity was below the speed of light, we would always still be able to see light coming to us from the area of the origin point. With a random initial velocity distribution the fastest half of galaxies would be spread over a volume 4x bigger than that of the slowest half of galaxies. That means galaxy density in th direction of the origin point would be something like 4x higher that it is in directions away from the origin point. But as far as we can tell galaxy and galaxy cluster distribution is even. Then there's the cosmic microwave background. If this is the after glow of the big bang, we wouldn't see it. All the radiation would have spread out in a shell at the speed of light. So we'd need tome other explanation for the CMB and why it happens to have all the exact characteristics we predicted for a big bang after glow.

scotty79 3y ago

From each other. Bigbang that has a center is perfectly consistent with Hubble law. Imagine that the spacetime is flat, there's no gravity and clumps of matter (galaxies) initially had random velocities. The faster the galaxy was, the further is went till now. What is a bit surprising, is that if you do the math in such universe and compare distances from any galaxy to others and their relative velocities you'll get Hubble law exactly. For every galaxy the distance to any other galaxy is directly proportional to relative velocity between them and they appear to be moving directly away from each other. So even though this big bang had epicenter you can't really tell that it had it or where it was if you can't see the edge. And if our observable universe is small when compared with the size of the entire universe we have a low chance of being near the edge. Try it. You can do it on paper or do a little computer simulation and measure. I did it few years ago. I was quite surprised with the results. Especially with the direction of the relative velocity vectors. They always face directly away from one galaxy to any other.

gpderetta 3y ago
> From each other. Bigbang that has a center is perfectly consistent with Hubble law. So we see all distant galaxies moving radially away us. Are you saying that the big bang happened to be centered, of all the universe, around the Milky Way?
- scotty79 3y ago
  No. I'm saying that if big bang imparted random velocities on galaxies that were all initially in the same spot (epicenter) then tofay in the reference frame of any galaxy (also Milky Way) it will look like every other galaxy is moving away from it radially with the velocity proportional to the current distance from it. No one will be able to tell where the actual epicenter was if they are too far from the edge to see it. This holds for the universe without gravity, with flat spacetime, Newtonian physics, with three, two or one spatial dimension. Seriously, try to simulate such toy universe. Or just sit down with a piece of paper and consider variant with one spatial dimension. It's primary school math and physics. You'll get Hubble law.
  - simonh 3y ago
    You’re right about the distribution of the velocities of galaxies. The problem would be the distribution of the density of galaxies. Even in an instantaneous snapshot of the state of the universe now, there would be more galaxies in the direction of the epicentre from us, say assuming we had average velocity. To see an even distribution of actual galaxies (not their velocities) in the sky we’d need to have zero initial velocity. But also light would still take time to travel. Our view of galaxies 13bn light years away would still be of them as they were 13bn years ago in the very early universe. Again unless we happened to have zero initial velocity, we’d see all the distant galaxies clustered together in one spot in the sky.
    - scotty79 3y ago
      We don't know of the distribution of galaxies in our universe is exactly equal. And the exact amount of any anisotropy is highly dependant on how small fraction of the whole universe is observable by us and how far are we from epicenter and the edge. 13bn light-years worth of galaxies might be too small sample to notice any asymetry with our current equipment. What if that's just one billionth of all the matter thrown apart by the big bang? Universe did not necessarily start from a point, just from a very energetic state where the all matter we can see now was fairly close together.
      - simonh 3y ago
        We can see the relative motion of galaxies up to the edge of the observable universe from their red shift. So we know they were all 'in one place' 13 bn years ago. It also means we know what the distribution of initial vectors must have been in this scenario for these galaxies. So either the conventional view is correct and it's space that has expanded moving the galaxies further apart. This is consistent with a potentially infinite universe. Alternatively all the galaxies we can see were clustered together in a tight lump 13bn years ago and flew apart. If the latter is the case the origin point would still be visible to us. As long as our initial random velocity was below the speed of light, we would always still be able to see light coming to us from the area of the origin point. With a random initial velocity distribution the fastest half of galaxies would be spread over a volume 4x bigger than that of the slowest half of galaxies. That means galaxy density in th direction of the origin point would be something like 4x higher that it is in directions away from the origin point. But as far as we can tell galaxy and galaxy cluster distribution is even. Then there's the cosmic microwave background. If this is the after glow of the big bang, we wouldn't see it. All the radiation would have spread out in a shell at the speed of light. So we'd need tome other explanation for the CMB and why it happens to have all the exact characteristics we predicted for a big bang after glow.