The theory of Big Bang nucleosynthesis, which predicts the observed relative abundance of the chemical elements in observations of the recent universe. Higher numbers of baryons in the early universe should produce higher ratios for helium, lithium, and heavier elements relative to hydrogen.[6][7] Agreement with observed abundances requires that baryonic matter makes up between 4–5% of the universe’s critical density.
Thanks. I guess my biggest confusion was that this could all be measured with such precision back then. To the point that astrophysicists in the 19th century could confidently deduce that something was wrong.
It doesn’t take incredible precision - just a lot of observations, and by that point, there was a devoted group of people taking an interest; one reason for the interest was that in the earlier 19th century, paying attention to the orbit of Uranus allowed for the discovery of Neptune (and that was harder, I think, because Neptune’s orbital period is so long); Mercury has been observed in detail for centuries, and completes an orbit every 88 days, so there was a lot of data to analyze
Short version: Most orbits aren’t perfect circles, but ellipses (a circle is still allowed, but out of all of the ellipses possible, a perfect circle is unlikely). And Mercury’s orbit is more elliptical than most planets. So the orbit has a long direction and a narrow direction. If it were just a matter of Newtonian gravity, and there were only the Sun and Mercury, that long direction would point in a particular direction, and it would keep on pointing in that same direction forever. But the existence of other planets messes that up, and causes the long direction to change very slightly over very long time periods. Well, astronomers of the late 19th century were able to measure this effect, and they were also able to calculate how much effect the known planets would account for, and the numbers didn’t match up: There was still a discrepancy of around a hundredth of a degree change per century (like I said, it changes only very slightly). So the original explanation was some other, unknown planet, but then it turned out that you could also explain it if gravity wasn’t quite precisely Newtonian.
In the very early Universe, when protons and neutrons were still new, most of those protons and neutrons stayed separate, but some of them joined into various isotopes of hydrogen, helium, and lithium. How many of them did this, into what isotopes, depended on the density of baryons at the time. And those isotopes are still around, and (aside from Helium 4), were almost entirely formed in these early-Universe processes, so we can see from the relative abundances just how common baryonic matter was back then. Which in turn tells us how common it is now. And the figures you get from the total amount of baryonic matter aren’t close to the amount of dark matter we observe. Sure, there’s some baryonic dark matter, but not enough.
Incidentally, “normal” black holes, the kind formed from the collapse of stars, also figure into this total, because even though they’re not baryonic matter now, they were, back in the time when those isotopes were being formed. So stellar black holes can’t explain the dark matter, either. But primordial black holes might predate the time of protons and neutrons.
Then again, though, we already have that, too, because while baryonic matter isn’t at the same level as the exotic dark matter, it’s not a negligible fraction, either. If one kind of matter (baryonic) can be that big a fraction of the total, maybe others can be, too. And maybe there is some reason that different kinds of matter should be at comparable abundances.
The CMB gets cooler and weaker as it redshifts, and while that seems like a slow process to us, it’s lightning-fast compared to the timescales involved in black hole evaporation.
This sounds like basically a description of the process by which primordial black holes might have formed in the first place. So, maybe?
And this is an argument in favor of particle-based solutions and in disfavor of PBH solutions. Baryonic and non-baryonic particles appearing at comparable abundances strains no sense of naturalness, and is even expected in many theoretical descriptions. Doing the same with baryonic matter and PBHs relies on the luck of a random draw as far as we understand it today.
And just to clarify something: does the size of the final burst of energy that occurs when the black hole crosses the threshold and evaporates away for good (it’s a chain reaction since the smaller it gets the faster it loses mass) depends on the mass of the initial black hole?
Nope, a black hole has no properties other than mass, angular momentum, electric charge, and magnetic charge. How it got to its current state is irrelevant, whether it was born that size, born smaller and then ate up, or born larger and then evaporated down.
That final burst might (probably would) look different depending on angular momentum and charge (if those properties aren’t lost before the end), but not based on initial mass.
Of note: The cosmic microwave background data is significantly more constraining here than Big Bang nucleosynthesis (which is why I list it first in my bulleted list further up). Generally the BBN analyses rely on CMB data as input as well, both to take advantage of the extra precision and also because the information is fairly complementary (with CMB providing information on the photon and baryon densities and BBN providing information on the ratio of those (sort of)).
As I was told it, if Mercury didn’t exist and we were forced to use Venus to try to prove GR, we still (as of a couple of decades ago) couldn’t do it, because Venus’s orbit is so very close to circular that error bounds on the location of the observed perihelion/aphelion are so much larger than the expected precession amount.
Not a physicist, but IIRC it’s something like 1% of the Sun’s output in the last moments. So impressive for something that size, but nowhere near a supernova.
And yes, since black holes are such simple objects they will all decay the same way.
Venus also has a smaller effect due to being further from the Sun, which also rules out Mars (which has the second-largest eccentricity). And comets aren’t any good, either, because we have such a short time when we can observe them.
Really? No, we don’t, because the final stages of black hole evaporation must be in the range where quantum gravity is involved. It’s even possible, for instance, that black holes don’t evaporate all the way, and that some quantum gravity phenomenon causes them to become stable and leave behind some sort of relic. But we can make guesses based on extrapolating what we do know, and we can set bounds from assuming that quantum gravity still respects things like conservation of mass, and those bounds and guesses still put it at well below supernova levels. We’d probably see one if it were within the Solar System, but not much beyond that.
OK, that all makes a lot of intuitive sense; I’m not sure where I got the idea that it was supposed to be variable, but I couldn’t figure out how it would be (since a half solar mass black hole that formed at that size should presumably be identical to a half solar mass black hole that formed at solar mass and evaporated that far). It makes a lot more sense if there is no difference.
Though the name suggests ‘no’ … is non-baryonic dark matter (aside from black holes) the kind of stuff a human could interact with? It would be invisible (?), but could it make sound waves? Could a bit of it be collected and laid in one’s palm? I assume that the gravitational effects of a large quantity of non-baryonic matter could be felt in much the same way as a large mass of baryonic matter (e.g. the Earth).
Well, there are the Jupiter family comets, of which there are some 400. They all have orbital periods of less than 20 years, so we see them go past the Sun fairly often. The problem with using them for proving General Relativity is that comets have another source of acceleration besides gravity. That source is the gas jets caused by ices on the comet sublimating when they get near the Sun. Those can change their orbital period by several days from one orbit to another. That will totally overwhelm the changes in their orbits due to GR.
To reconcile this with the fact that limits do come from non-observation: The abundance of evaporating PBHs is constrained by looking for the gamma rays and positrons that would be emitted during the evaporation process. No suitable excess rate of such radiation is seen galactically or extragalactically, which sets the upper limit on the abundance.
No, you couldn’t touch it, hold it, hear it, or anything like that. All of that requires electromagnetic interactions. You’re correct that gravity is gravity all the same for it.
The very weak (at best) interactions that dark matter can have with normal matter is how experiments can try to “see” dark matter particles directly in sensitive laboratory settings.
Given those properties … any reason to think that, say, the inner solar system is not rife with non-baryonic matter? As the ISS orbits Earth, is it presumably passing through (without interaction) a bunch of non-baryonic matter?
I really should probably let the experts answer this, but given that there is about 5 times as much dark matter as there is baryonic matter in the universe, I would say it is extremely likely that the ISS passes through it every day. I would go even further and state that is likely some of it is passing through you and the rest of the earth every day.
Sortof, like how in any given second, trillions of neutrinos pass through your body without you noticing because outside of gravity, it just doesn’t really interact with normal matter. (Note neutrinos have already been ruled out as a dark matter candidate, but you would have to ask the experts why. (Something about it being too fast IIRC.)
Yes, as are you. The ISS in space and you in your chair are moving at essentially the same speed through the galactic dark matter.
Back of the envelope: if dark matter consists of particles that are similar in mass to a proton, then around 1012 dark matter particles pass through your body every second.
(The mass of the particle matters since it’s the dark matter energy density (or mass density) that is measured well, and that could mean many lighter particles or fewer heavier particles, up to a point.)