5 ms·

Yeah, that's a valid idea; we considered that about 40 years ago and concluded it doesn't work. Yes, even if the amounts were perfectly equal, you would expect

by knzhou 7y ago

Yeah, that's a valid idea; we considered that about 40 years ago and concluded it doesn't work. Yes, even if the amounts were perfectly equal, you would expect some regions with net matter or antimatter content. But when you run the numbers, you expect the sign to flip from place to place (which would produce matter-antimatter annihilations at the boundaries, which are not observed) and for the magnitude of the local imbalances to be far lower (by orders of magnitude of orders of magnitude) versus what is observed.

rtkwe 7y ago
Weird question but when did antimatter start existing, ie when would the matter-antimatter annihilation happen? Couldn't it just be that it happened early enough that the universe was still densely packed enough for essentially a 1-1 annihilation to happen? Then any tiny imbalance towards either side would result in a universe entirely composed of one type of matter. I'm sure someone's already thought of that and shot it full of holes though. Weird side idea could this huge burst of energy that would be released from that be the Big Bang?
- deleted 7y ago
  [deleted]
- kstrauser 7y ago
  For that matter, could such an annihilation provide some of the energy of expansion? (I'm asking totally in ignorance. This stuff fascinates me but I don't know nearly as much about it as I wish I did.)
  - pdonis 7y ago
    > could such an annihilation provide some of the energy of expansion? No. The universe was already rapidly expanding before the annihilation period. The expansion was slowing down all during that period.
    - kstrauser 7y ago
      Ah, got it. Thanks!
- evanb 7y ago
  That actually IS the basic idea---there was almost a 1:1 annihilation. But it's not 1:1 exactly (look around you, matter everywhere). We quantify the baryon asymmetry as η = (n_baryons - n_antibaryons) / n_photons where the n_species are the number densities of those species. Since baryon number is additively conserved, the asymmetry now reflects the asymmetry in the past. From WMAP we estimate η ~ (6±0.25)E-10. The question is: why is this SO damn small? There was alllllllmost a perfect cancellation. But not quite. What gives? On the other hand, it's not QUITE as early as you hope---the anti/baryon annihilation is to light particles and radiation, not the negative pressure needed to cause inflationary Big Bang cosmology.
  - _0ffh 7y ago
    Doesn't an /almost/ perfect cancellation actually make the question "Why is it so close?" not "Why is there an asymmetry at all?". If I tossed a million coins and ended up 499,999 heads and 500,001 tails I wouldn't be surprised by the difference, but by it's small size.
    - knzhou 7y ago
      Indeed, that's exactly how we used to talk about it: if you can just postulate some initial asymmetry, the question to answer is why it's small. We don't think this way anymore because we're fairly sure inflation occurred, and its exponential expansion rapidly dilutes away any initial asymmetry. So we need a way to generate asymmetry after inflation ends.
- pdonis 7y ago
  > when did antimatter start existing The same time matter started existing. According to our best current models, that was at the end of inflation, when the energy that had been stored in the inflaton field (the field that was driving inflation) was transferred to the Standard Model fields (leptons, quarks, and gauge bosons). This process is called "reheating" (somewhat of a misnomer since it was really the first "heating" of the SM fields). The most natural expectation is that reheating would have populated matter and antimatter fields (i.e., quarks-antiquarks, leptons-antileptons) equally. Then, as the universe expanded and cooled, the matter and antimatter would gradually annihilate, producing radiation (mainly photons). If matter and antimatter fields had really been populated equally at reheating, this process would have left nothing but photons within a fairly short time (a fraction of a second) after the Big Bang. "Local imbalances" might have made that time a somewhat larger fraction of a second, but it is not at all plausible that they could have lasted for billions of years and allowed a region the size of our observable universe to have essentially no antimatter at all but billions of galaxies' worth of matter. Since our observable universe does have essentially no antimatter and billions of galaxies' worth of matter, one of two things must have been the case: (1) reheating did not populate the matter and antimatter fields equally, or (2) there was some asymmetry in the interactions involved that created a slight excess of matter over antimatter. As I understand it, most physicists believe (2) to be the case, although it is not known exactly what the asymmetry was. > could this huge burst of energy that would be released from that be the Big Bang? No. The energy had to be put in to the matter and antimatter fields first. See above.
  - eveningcoffee 7y ago
    What interest me in this context is that when you say in fraction of a second then do you mean the second how we sense it or is this second somehow longer because of the huge gravity? Or was there gravity at all?
    - pdonis 7y ago
      > do you mean the second how we sense it I mean it in the sense of an SI second, which is defined in terms of a certain number of cycles of light of a certain frequency that comes from a certain energy level transition in cesium atoms. > is this second somehow longer because of the huge gravity? No. The concept of "gravitational time dilation" is not applicable in the circumstances we are talking about. > was there gravity at all? "Gravity" is an ambiguous term. On average, the universe is homogeneous and isotropic (and was even more so in the very early universe), which means that on average the matter in the universe exerts no net gravitational force on any particular piece of matter, such as us here on Earth or a tiny parcel of matter in the early universe. In that sense there is "no gravity" in a homogeneous, isotropic universe. However, the presence of matter and energy certainly affects the spacetime geometry. In that sense, there is "gravity" present.

by knzhou 7y ago

rtkwe 7y ago

Weird question but when did antimatter start existing, ie when would the matter-antimatter annihilation happen? Couldn't it just be that it happened early enough that the universe was still densely packed enough for essentially a 1-1 annihilation to happen? Then any tiny imbalance towards either side would result in a universe entirely composed of one type of matter. I'm sure someone's already thought of that and shot it full of holes though. Weird side idea could this huge burst of energy that would be released from that be the Big Bang?

deleted 7y ago
[deleted]
kstrauser 7y ago
For that matter, could such an annihilation provide some of the energy of expansion? (I'm asking totally in ignorance. This stuff fascinates me but I don't know nearly as much about it as I wish I did.)
- pdonis 7y ago
  > could such an annihilation provide some of the energy of expansion? No. The universe was already rapidly expanding before the annihilation period. The expansion was slowing down all during that period.
  - kstrauser 7y ago
    Ah, got it. Thanks!
evanb 7y ago
That actually IS the basic idea---there was almost a 1:1 annihilation. But it's not 1:1 exactly (look around you, matter everywhere). We quantify the baryon asymmetry as η = (n_baryons - n_antibaryons) / n_photons where the n_species are the number densities of those species. Since baryon number is additively conserved, the asymmetry now reflects the asymmetry in the past. From WMAP we estimate η ~ (6±0.25)E-10. The question is: why is this SO damn small? There was alllllllmost a perfect cancellation. But not quite. What gives? On the other hand, it's not QUITE as early as you hope---the anti/baryon annihilation is to light particles and radiation, not the negative pressure needed to cause inflationary Big Bang cosmology.
- _0ffh 7y ago
  Doesn't an /almost/ perfect cancellation actually make the question "Why is it so close?" not "Why is there an asymmetry at all?". If I tossed a million coins and ended up 499,999 heads and 500,001 tails I wouldn't be surprised by the difference, but by it's small size.
  - knzhou 7y ago
    Indeed, that's exactly how we used to talk about it: if you can just postulate some initial asymmetry, the question to answer is why it's small. We don't think this way anymore because we're fairly sure inflation occurred, and its exponential expansion rapidly dilutes away any initial asymmetry. So we need a way to generate asymmetry after inflation ends.
pdonis 7y ago
> when did antimatter start existing The same time matter started existing. According to our best current models, that was at the end of inflation, when the energy that had been stored in the inflaton field (the field that was driving inflation) was transferred to the Standard Model fields (leptons, quarks, and gauge bosons). This process is called "reheating" (somewhat of a misnomer since it was really the first "heating" of the SM fields). The most natural expectation is that reheating would have populated matter and antimatter fields (i.e., quarks-antiquarks, leptons-antileptons) equally. Then, as the universe expanded and cooled, the matter and antimatter would gradually annihilate, producing radiation (mainly photons). If matter and antimatter fields had really been populated equally at reheating, this process would have left nothing but photons within a fairly short time (a fraction of a second) after the Big Bang. "Local imbalances" might have made that time a somewhat larger fraction of a second, but it is not at all plausible that they could have lasted for billions of years and allowed a region the size of our observable universe to have essentially no antimatter at all but billions of galaxies' worth of matter. Since our observable universe does have essentially no antimatter and billions of galaxies' worth of matter, one of two things must have been the case: (1) reheating did not populate the matter and antimatter fields equally, or (2) there was some asymmetry in the interactions involved that created a slight excess of matter over antimatter. As I understand it, most physicists believe (2) to be the case, although it is not known exactly what the asymmetry was. > could this huge burst of energy that would be released from that be the Big Bang? No. The energy had to be put in to the matter and antimatter fields first. See above.
- eveningcoffee 7y ago
  What interest me in this context is that when you say in fraction of a second then do you mean the second how we sense it or is this second somehow longer because of the huge gravity? Or was there gravity at all?
  - pdonis 7y ago
    > do you mean the second how we sense it I mean it in the sense of an SI second, which is defined in terms of a certain number of cycles of light of a certain frequency that comes from a certain energy level transition in cesium atoms. > is this second somehow longer because of the huge gravity? No. The concept of "gravitational time dilation" is not applicable in the circumstances we are talking about. > was there gravity at all? "Gravity" is an ambiguous term. On average, the universe is homogeneous and isotropic (and was even more so in the very early universe), which means that on average the matter in the universe exerts no net gravitational force on any particular piece of matter, such as us here on Earth or a tiny parcel of matter in the early universe. In that sense there is "no gravity" in a homogeneous, isotropic universe. However, the presence of matter and energy certainly affects the spacetime geometry. In that sense, there is "gravity" present.