A bit of a tangent, but I don’t think the claim that we have sub-percent knowledge a femtosecond after the Big Bang is justified. This is at the tail end of reheating, whose mechanism is currently poorly understood.
Welcome to the forum @PdotdQ. Tell us about yourself .
Inflation cools the Universe by an extraordinary amount. At the end of inflation the Universe is heated back up by a process called reheating. During reheating, the inflaton field responsible for inflation decays into mundane particles.
The mechanism of reheating is poorly understood because:
- It depends on the particular inflation scenario that precedes it. Reheating is highly model dependent even within the framework of inflation. This uncertainty is compounded by issues of the inflation framework itself.
- Presumably during the reheating epoch more matter is produced than antimatter. This asymmetry is totally unexplained and cannot be explained by Standard Model Physics. This suggests that whatever mechanism behind reheating is currently beyond known physics.
Welcome to you and thanks for the information! It is interesting to read and (this is not always the case, here) accessible!
That makes two MIT physicists @Patrick, hehe. @PdotdQ, I think you just made his week. He is gonna be asking you lots and lots of questions I predict. To warn you though, he is not too happy with Catholicism right now. He might try and proselytize. For totally understandable reasons, @Patrick is going through an atheist version of a “street-preacher phase” regarding Catholicism.
I’m a biologist, but I’ve learned a lot from Ethan Siegel and Sabine Hossenfelder. Do you have thoughts on Sabine’s accidental theory of modified gravity?
Unless Daniel moved, we were at Harvard instead of MIT
I have not read Sabine’s paper so I do not know how justifiable it is. However I have read McGaugh et al.'s paper that was the impetus for her theory in great detail.
As most astrophysicists, I subscribe to the “slightly-perturbed”-Lambda-CDM model, i.e. the idea that while Lambda-CDM got some details wrong, it is pretty much correct (especially at cosmological scales).
This past few years, our worldview has been rocked by a slew of results showing that perhaps Lambda-CDM is not as accurate as we thought. The strongest blow was dealt by the McGaugh et al. paper, which shows that certain properties of galaxies are better fitted by MOND-type theories.
Nevertheless, a full reconciliation between what I call “slightly-perturbed”-Lambda-CDM and McGaugh et al.'s result is possible, and I do not think that the blow was strong enough to dethrone the reigning theory.
Oops. Sorry. I’ve have committed a grave offense. He has corrected me on this before. I am so sorry. We now have two HARVARD physicists. By the way, do you know Owen Gingerich at Harvard who does history of science and is a leading scholar of astronomy?
Yes, for four years his office is right above mine. I’m sure he is sick and tired of my voice (we graduate students argued a lot).
It is interesting primarily because it is a no-parameter model that fits the data. That is interesting, and surprising.
I also find it mind bending that she argues that the Bullet Cluster is strong evidence against Dark Matter. Of course, in this, she is exactly opposite of Ethan Seigel. I’m really curious who is ultimately going to be shown correct.
This is my assessment. Do you think it is off?
I’d take an MIT physicist over a Harvard Business School grad any day…
Depending on who you ask, the Bullet Cluster is evidence for and against dark matter. While I think that the Bullet Cluster is evidence for dark matter, this is not my field and my knowledge might not be up to date. Nevertheless, here are my arguments written in the style of St. Aquinas.
Is the Bullet Cluster evidence for dark matter?
Objection 1: The reason the Bullet Cluster is thought to be evidence of dark matter is because of gravitational lensing detection of unseen mass at the sides of the colliding clusters. This is thought to only be explainable by dark matter theories. However, relativistic extensions of MOND is capable of reproducing the gravitational lensing detection.
Objection 2 (Sabine’s objection): Different theories predicts clusters with different velocities. Numerical simulations using dark matter theories (Lambda-CDM) shows that it is extremely unlikely for two clusters to collide with such high relative velocities as is seen in the Bullet Cluster.
On the contrary:
The Bullet Cluster provides good evidence for dark matter because the existence of dark matter explains the behavior of the different components of the Bullet Cluster well.
I answer that:
The main reason the Bullet Cluster provides good evidence for dark matter is not because of the existence of the “unseen mass”, but due to its behavior. In the dark matter picture, the original two clusters in the collision posses two main components: gas and dark matter. When it collides, the dark matter components pass through each other, forming the “wings” of the cluster. The gaseous components cannot pass through each other and got stuck in the middle.
Reply to Objection 1:
While it is true that relativistic extensions of MOND can produce the gravitational lensing signatures, it is not good enough for two reasons: first, relativistic MOND still require ~100% the mass of the cluster to be “unseen”. While this is less than the ~1000% that naive MOND requires, it is still substantial. Further, as was mentioned previously, this “unseen” matter cannot be mundane gas that is too dim to see, because they have to pass through each other.
Reply to Objection 2:
This claim depends on the various inputs of the numerical simulations, all of which are less than certain. Most numerical cosmological simulations rely on ad-hoc prescriptions to model processes that are beyond the computational resolution. The robustness of this claim is therefore still suspect. Further, to my knowledge there is no cosmological simulations using MOND-type theories that can produce these high velocity cluster collisions (although there is some theoretical evidence showing that this is possible in MOND-type theories).
I do not understand this. How can there be 1000% of the mass? Do you mean 10x the observable mass must be unobservable?
11 posts were split to a new topic: Dark Matter as an Emergent Particle
Awesome, it’s such a pleasure to see you here! As you can see we have some people very curious about cosmology, astrophysics, and science in general here, as well as some really unique discussions about theology and science, with excellent moderation and a very interesting set of forum members from all theological backgrounds I can think of. (Actually, having more Catholic representation would be really awesome.) We have some people here who are fans of Aquinas as well - I think @Eddie and @jongarvey are in that camp.
Everyone, @PdotdQ is a really outstanding astrophysicist who is very knowledgeable about cosmology, astrophysics, general relativity, and similar subjects - and in my experience, really good at explaining it to a general audience as well! @Patrick, here’s a chance to ask your technical questions about the CMB or Big Bang cosmology if you have any.
See, we do real science here at PS .
Maybe not, but like other asymmetries such as stereochemistry in biology, highly consistent with a teleological understanding of the world. That would be so even if a mechanism were found.
We were doing so well discussing physics, the origin of the universe, and then you had to bring HIM into the discussion.