3 ms·

> "Intergalactic void is so empty that next galaxy could be made of anti-matter (it probably isn't) and we couldn't tell." This isn't actually true, there's en

> "Intergalactic void is so empty that next galaxy could be made of anti-matter (it probably isn't) and we couldn't tell." This isn't actually true, there's enough matter in intergalactic space that if there were galaxies made of anti-matter anywhere in the visible universe the interface between regular and anti-matter would shine more powerfully than the galaxy, and have a very characteristic spectrum as well. Nothing of the sort has been observed.

Aelinsaar 10y ago
To be fair, there are probably a few places such a system could hide from a given observer, but that starts to place some pretty ridiculous restraints on the whole endeavor.
scotty79 10y ago
Are you sure? I read something written by a physicist that we really can't tell that with certainty about remote galaxies. Besides, there might be even less dense areas of space than intergalactic void, for example between clusters of galaxies. Also composition of the void at the boundary might vary smoothly between matter and antimatter and mostly just photons in between. If there are areas of matter and areas of antimatter in the universe then the space between them in young universe would be dominated by energy (photons) pushing them apart and creating natural separation.
- Florin_Andrei 10y ago
  http://www.scientificamerican.com/article/how-do-we-know-that-dista/ http://www.scientificamerican.com/article/how-do-we-know-tha... TLDR: It's not a sure thing in a strict sense, but it's very unlikely that there are anti-matter pockets in the observable universe.
  - Retric 10y ago
    Well prefixing that by saying observable universe. We can't really say much about the non observable universe.
    - dpark 10y ago
      Sure. The unobservable universe might be filled with antimatter and marshmallows. It's pretty unlikely though since everything indicates that the universe is generally pretty homogeneous.
  - scotty79 10y ago
    The article you linked says that whole clusters or whole superclusters could be made of antimatter. The problem is just how to fit them to current state of big bang model. I'd say that annihilation is sufficient mechanism to create sufficient separation between matter and antimatter. It produces energy that could keep superclusters apart and if all that happened before the microwave background we really couldn't tell.
    - raattgift 10y ago
      Your idea is something which has been studied pretty intensively for decades; it's that there are huge and distinct regions of antimatter with vanishing baryon number B = 0 and that these have existed since at least the start of early structure formation, thus hoping to avoid (highly redshifted) signals in the cosmic diffuse gamma background. That the anisotropies in the CMB are small puts sufficient limits on structure formation in the earlier universe that our observations (even at COBE scale, let alone from recent observatories) preclude your idea. AMS 02 results to date essentially preclude the wiggle-room of very large antimatter structures at large comoving distances which have "always" been spacelike separated from very large matter structures. BAO observations also put strong limits on the existence of large antimatter structures; it is very very hard to think of ways that gravitational collapse of antimatter can avoid leaving obvious traces on the Lyman-\alpha forest, without entirely overthrowing the Copernican principle. As far as we can tell -- and there are thirty years of observations probing exactly this question -- there are zero antimatter AGNs and zero early superluminous antimatter supernovae in our sky; if they exist at all they are outside our Hubble volume, or there is a strongly Earth-focused conspiracy about the directions in which AGN relativistic jets point and the alignments on which they are filamented, which is also in tension with the Copernican principle, and that still does not explain the absorption lines on early SN emissions. This does not preclude other Hubble volumes filled with antimatter or with annihilation products; most models of inflation lead to many many Hubble volumes. However the observations to date effectively preclude much antimatter, and essentially all large structures of antimatter, in our own Hubble volume. There are some further arguments and an overview of many years worth of the large-antimatter-filled-regions-in-our-universe arguments in astro-ph/9707087 (Glashow et al., 1998) It was never a stupid argument, but it received a lot of attention in the past couple of decades, and is now essentially resolved. (There's an overview and citations of more recent discussion in part II of https://arxiv.org/abs/1204.4186v2 https://arxiv.org/abs/1204.4186v2 although the paper also discusses other ideas about "why isn't there more antimatter in our Hubble volume" and advances a pretty speculative idea of the authors' own.)
cperciva 10y ago
the interface between regular and anti-matter would shine Assuming, of course, that matter and antimatter came into contact rather than being separated by a "neutral zone". If the gravitational force between matter and antimatter were repulsive -- and as far as I'm aware nobody has ever succeeded in measuring its sign -- then matter and antimatter might coexist on intergalactic scales without coming into contact. (Even more speculative: If antimatter also repelled itself, the resulting "pressure" would have sign consistent with both the "dark matter" which results in elevated galactic rotation rates and the "dark energy" which pushes galaxies apart.)
- Retra 10y ago
  Galaxies come into contact regularly though, and we don't have any evidence that they interact in such a way.
  - cperciva 10y ago
    Galaxies which gravitationally attract come into contact regularly.
- akiselev 10y ago
  If the gravitational force between the two is repulsive, wouldn't that imply negative mass because of what we know of the Higgs field? Then we'd be deep into exotic matter territory and theoretically, even a small amount can warp space time very differently than regular matter so at galactic scales, shouldn't we see some pretty extreme evidence of this?
  - cperciva 10y ago
    Given how little we know about the Higgs -- namely, that we're pretty certain that at least one particle exists which probably behaves in the same manner as a Higgs boson would on several tests -- I'd hesitate to draw conclusions on the basis of how a particular model suggests that the Higgs field behaves.
- InclinedPlane 10y ago

Aelinsaar 10y ago

To be fair, there are probably a few places such a system could hide from a given observer, but that starts to place some pretty ridiculous restraints on the whole endeavor.

scotty79 10y ago

Are you sure? I read something written by a physicist that we really can't tell that with certainty about remote galaxies. Besides, there might be even less dense areas of space than intergalactic void, for example between clusters of galaxies. Also composition of the void at the boundary might vary smoothly between matter and antimatter and mostly just photons in between. If there are areas of matter and areas of antimatter in the universe then the space between them in young universe would be dominated by energy (photons) pushing them apart and creating natural separation.

Florin_Andrei 10y ago
http://www.scientificamerican.com/article/how-do-we-know-that-dista/ http://www.scientificamerican.com/article/how-do-we-know-tha... TLDR: It's not a sure thing in a strict sense, but it's very unlikely that there are anti-matter pockets in the observable universe.
- Retric 10y ago
  Well prefixing that by saying observable universe. We can't really say much about the non observable universe.
  - dpark 10y ago
    Sure. The unobservable universe might be filled with antimatter and marshmallows. It's pretty unlikely though since everything indicates that the universe is generally pretty homogeneous.
- scotty79 10y ago
  The article you linked says that whole clusters or whole superclusters could be made of antimatter. The problem is just how to fit them to current state of big bang model. I'd say that annihilation is sufficient mechanism to create sufficient separation between matter and antimatter. It produces energy that could keep superclusters apart and if all that happened before the microwave background we really couldn't tell.
  - raattgift 10y ago
    Your idea is something which has been studied pretty intensively for decades; it's that there are huge and distinct regions of antimatter with vanishing baryon number B = 0 and that these have existed since at least the start of early structure formation, thus hoping to avoid (highly redshifted) signals in the cosmic diffuse gamma background. That the anisotropies in the CMB are small puts sufficient limits on structure formation in the earlier universe that our observations (even at COBE scale, let alone from recent observatories) preclude your idea. AMS 02 results to date essentially preclude the wiggle-room of very large antimatter structures at large comoving distances which have "always" been spacelike separated from very large matter structures. BAO observations also put strong limits on the existence of large antimatter structures; it is very very hard to think of ways that gravitational collapse of antimatter can avoid leaving obvious traces on the Lyman-\alpha forest, without entirely overthrowing the Copernican principle. As far as we can tell -- and there are thirty years of observations probing exactly this question -- there are zero antimatter AGNs and zero early superluminous antimatter supernovae in our sky; if they exist at all they are outside our Hubble volume, or there is a strongly Earth-focused conspiracy about the directions in which AGN relativistic jets point and the alignments on which they are filamented, which is also in tension with the Copernican principle, and that still does not explain the absorption lines on early SN emissions. This does not preclude other Hubble volumes filled with antimatter or with annihilation products; most models of inflation lead to many many Hubble volumes. However the observations to date effectively preclude much antimatter, and essentially all large structures of antimatter, in our own Hubble volume. There are some further arguments and an overview of many years worth of the large-antimatter-filled-regions-in-our-universe arguments in astro-ph/9707087 (Glashow et al., 1998) It was never a stupid argument, but it received a lot of attention in the past couple of decades, and is now essentially resolved. (There's an overview and citations of more recent discussion in part II of https://arxiv.org/abs/1204.4186v2 https://arxiv.org/abs/1204.4186v2 although the paper also discusses other ideas about "why isn't there more antimatter in our Hubble volume" and advances a pretty speculative idea of the authors' own.)

cperciva 10y ago

the interface between regular and anti-matter would shine Assuming, of course, that matter and antimatter came into contact rather than being separated by a "neutral zone". If the gravitational force between matter and antimatter were repulsive -- and as far as I'm aware nobody has ever succeeded in measuring its sign -- then matter and antimatter might coexist on intergalactic scales without coming into contact. (Even more speculative: If antimatter also repelled itself, the resulting "pressure" would have sign consistent with both the "dark matter" which results in elevated galactic rotation rates and the "dark energy" which pushes galaxies apart.)

Retra 10y ago
Galaxies come into contact regularly though, and we don't have any evidence that they interact in such a way.
- cperciva 10y ago
  Galaxies which gravitationally attract come into contact regularly.
akiselev 10y ago
If the gravitational force between the two is repulsive, wouldn't that imply negative mass because of what we know of the Higgs field? Then we'd be deep into exotic matter territory and theoretically, even a small amount can warp space time very differently than regular matter so at galactic scales, shouldn't we see some pretty extreme evidence of this?
- cperciva 10y ago
  Given how little we know about the Higgs -- namely, that we're pretty certain that at least one particle exists which probably behaves in the same manner as a Higgs boson would on several tests -- I'd hesitate to draw conclusions on the basis of how a particular model suggests that the Higgs field behaves.
InclinedPlane 10y ago