3 ms·
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 a
by 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 agoDoesn'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 agoIndeed, 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.