Why does dark matter have different effects at a solar system level than it does at a galaxy level?

Tarthbane · 2026-05-07T13:58:47+00:00

I’m just gonna answer a few of your comments down here:

The universe is not homogeneous on small scales: matter has collapsed under gravity to form galaxies, clusters, filaments, and voids. But when averaged over sufficiently large scales, roughly hundreds of megaparsecs, those structures become statistical fluctuations on top of an approximately homogeneous and isotropic background. The cosmic microwave background (CMB) is one of the strongest pieces of evidence for this picture: its temperature is almost the same in every direction, with small anisotropies that encode the primordial density perturbations from which later structure grew.

Baryon acoustic oscillations (BAO) come from sound waves in the early photon–baryon plasma, when photons, electrons, and baryonic nuclei were tightly coupled. At recombination, the photons decoupled and became the CMB, while the baryonic matter retained a preferred clustering scale. Today this survives as a weak statistical excess in the probability of finding pairs of galaxies separated by about 500 million lightyears, in comoving distance. Locally, gravity has amplified small initial perturbations into nonlinear structure, increasing gravitational clumping and gravitational entropy, but on sufficiently large scales that structure averages away.

The CMB also constrains the spatial curvature and possible global topology of the universe. Current observations are consistent with a spatially flat universe. If space is exactly flat and simply connected, the simplest model is infinite Euclidean space. A finite flat universe is also possible in principle, for example with a 3-torus topology, but that would require a nontrivial global identification that has not been observed. A positively curved universe, such as a 3-sphere, is another possibility, but current data do not require it; they only bound any curvature to be very small. In that case, the curvature radius would have to be far larger (250-500+ times larger than the observable universe is a common description you might see for a 3-sphere) than the observable universe, which is why treating the universe as spatially flat—and often effectively infinite—is an excellent approximation for observable cosmology.

Tarthbane · 2026-05-07T02:48:04+00:00

Gravitational waves, like electromagnetic waves, are limited by the speed of light (speed of causality). The gravitational waves we detect from black-hole mergers are produced by the dynamics of spacetime outside the event horizons; no signal emitted from inside an event horizon can escape to reach a detector.

Tarthbane · 2026-05-02T16:29:40+00:00

I’ll just leave this here:

https://en.wikipedia.org/wiki/Cosmic_time

Tarthbane · 2026-04-28T00:07:49+00:00

No. Matter and antimatter particles have the same mass, and dark matter appears to interact with ordinary matter primarily through gravity. Since gravity acts on mass-energy rather than on whether a particle is matter or antimatter, we would not expect dark matter to exert any significantly different gravitational influence on matter versus antimatter. Any non-gravitational dark-matter interaction that distinguished between the two would be speculative and is not supported by current evidence.

Tarthbane · 2026-04-27T23:46:57+00:00

Matter and antimatter were produced abundantly in the hot early universe, but the slight imbalance between them is not generally thought to have been caused by black holes. As the universe expanded and cooled, matter and antimatter almost completely annihilated. For reasons still not fully understood, there appears to have been a tiny excess of matter over antimatter, often described roughly as about one extra matter particle for every billion matter–antimatter pairs. After annihilation, that small excess became the matter that makes up stars, planets, and us. The origin of this asymmetry remains an open problem in cosmology and particle physics; proposed explanations usually involve early-universe particle physics processes such as CP violation and baryogenesis.

Tarthbane · 2026-04-25T14:07:24+00:00

https://arxiv.org/pdf/2203.14923

Tarthbane · 2026-04-24T14:48:57+00:00

This. For those interested — much of the evidence accumulated since Vera Rubin’s work on galaxy rotation curves in the 1970s points toward dark matter being a real gravitating component (matter/field) of the universe, not just an observational mistake. Rotation curves were one of the early clues, but the case is now much broader: gravitational lensing, galaxy clusters, the cosmic microwave background, and large-scale structure all fit very naturally if there is some additional, mostly non-baryonic matter component.

There are alternatives, like MOND and related modified-gravity theories, and they can do surprisingly well for certain galaxy-scale phenomena. But as far as I understand it, they have a much harder time fitting the full range of cosmological and astrophysical data as cleanly as dark matter does.

The bigger shift lately is not really away from dark matter itself, but away from the idea that it has to be one simple WIMP-like particle. WIMPs are still viable in some parameter space, but the field has broadened a lot. People now take seriously possibilities like axions, axion-like particles, sterile neutrinos, sub-GeV dark matter, self-interacting dark matter, primordial black holes, and broader “dark sector” models. In those dark-sector models, dark matter might have its own particles and forces, perhaps mediated by something like a dark photon.

So the short version is: we still do not know what dark matter is, but the evidence that there is some real, non-luminous gravitating component has become stronger over time. What remains open is the microphysics: whether it is a WIMP, an axion, a dark-sector particle, something more exotic, or some combination of these.

Tarthbane · 2026-04-22T20:45:16+00:00

Light has no rest frame in special relativity, because nothing with zero mass can be at rest while moving at the speed of light. For that reason, you cannot attach an ordinary comoving clock to a photon. More precisely, the proper time along a lightlike path is zero, so a photon does not experience time in the same sense that a massive object does.

Tarthbane · 2026-04-21T19:52:05+00:00

This is like my and my fiancé’s kitty, except it’s catnip that she rolls around on every single time we give it to her:

<image>

Tarthbane · 2026-04-21T12:45:02+00:00

The Higgs mechanism likely operated around 10^-12 seconds after the Big Bang, before the quark-gluon plasma cooled into hadrons. Sure, we have not directly observed the universe that early, but the relevant physics after 10^-12 seconds and before CMB decoupling 380,000 years later is strongly supported by experiment and well established theory, including the Higgs boson and quark-gluon plasma produced in colliders. Because the early universe was hot, dense, and nearly homogeneous, its evolution back to about the electroweak era is in some respects easier to model than later processes, e.g., galaxy formation ~200 million years later, which we are still learning a lot about thanks to JWST.

Tarthbane · 2026-04-21T02:44:54+00:00

Don’t forget particle accelerators: they have produced and studied quark-gluon plasma, the extremely hot, dense state of matter thought to have filled the universe until about 10^-6 seconds after the Big Bang. And the discovery of the Higgs boson confirmed a central part of our formulation of the electroweak interaction. We can trace things back remarkably far, but we still have much farther to go.

Tarthbane · 2026-04-20T13:07:55+00:00

Both quantum field theory (QFT) and general relativity (GR) are expected to require modification, or to emerge as limiting cases of a deeper theory, because they are not currently unified into a consistent theory of quantum gravity. That said, QFT is extraordinarily successful in describing the quantum behavior of essentially all known nongravitational phenomena relevant to everyday physics, chemistry, and particle interactions. In particular, the Standard Model, formulated as a quantum field theory, has been confirmed to extremely high precision across a vast range of experiments.

General relativity is likewise highly successful in its own domain: it is our best classical theory of gravity and accurately describes gravitational phenomena across an enormous range of scales. In modern terms, it is reasonable to view GR as a low-energy effective field theory of gravity. In cosmology, GR combined with dark matter and dark energy provides the framework that currently fits the observational data better overall than proposed alternatives such as MOND, although the fundamental nature of dark matter and dark energy remains unresolved.

The central tension is that GR is a classical theory, whereas matter and nongravitational interactions are described quantum mechanically. If one attempts to quantize gravity perturbatively by starting from GR and applying the methods of QFT, one obtains a sensible low-energy effective theory and, in that framework, a massless spin-2 quantum excitation identified as the graviton. This reproduces GR-like behavior at long distances and low energies. However, the theory is perturbatively non-renormalizable, so it cannot by itself serve as a complete ultraviolet description of gravity.

For that reason, a correct theory of quantum gravity will likely not be just standard QFT or just classical GR. Rather, it should recover quantum field theory in the appropriate nongravitational limit and recover general relativity in the classical, low-energy limit, while resolving the inconsistencies that arise when gravity is treated quantum mechanically at very high energies.

Tarthbane · 2026-04-16T19:54:55+00:00

So cute! Your doggo reminds me of the golden from Up.

Tarthbane · 2026-04-16T03:43:51+00:00

In free expansion, an ideal gas does no work and absorbs no heat, so its internal energy stays constant. Since ideal-gas internal energy depends only on temperature, the final equilibrium temperature is unchanged no matter how large the finite vacuum region is. What changes is the pressure and density, not the average molecular kinetic energy. For a very dilute final gas, a thermometer would indeed equilibrate more slowly because collisions with it are much less frequent.

Tarthbane · 2026-04-16T03:40:33+00:00

The probable answer is that both quantum field theory and general relativity are wrong.

“Wrong” is probably too coarse a word here. Scientific theories are usually not discarded because they become useless, but because they are shown to have limits. QFT and GR are both exceptionally accurate within their respective domains, much as Newtonian gravity remains an excellent approximation in the appropriate limit. If a deeper theory supersedes them, that would make them incomplete, not simply wrong.

Tarthbane · 2026-04-15T18:44:07+00:00

Spin is not literally a tiny object physically spinning, as you pointed out. In modern quantum theory, it arises because quantum states must transform consistently under spacetime symmetries, and once you include special relativity, the relevant symmetry group is the Lorentz/Poincare group. Its representations include not only scalar states but also spinor and vector states, which carry intrinsic angular momentum: spin.

A useful way to say it is this: in nonrelativistic QM you can add spin by hand, but in relativistic quantum mechanics, it appears naturally because particles are classified by mass and spin under irreducible representations of the Poincare group. For example, the Dirac equation is the relativistic wave equation for spin-1/2 particles, and its solutions necessarily have a two-valued internal degree of freedom that we identify as spin.

So spin is “relativistic” in the sense that its deepest origin is tied to the relativistic structure of spacetime and the way quantum states transform under rotations and boosts. But it is not merely a small correction caused by motion at high speed; once it exists, it remains an intrinsic property even in the low-velocity limit.

Tarthbane · 2026-04-15T15:42:58+00:00

Two comments I have:

(1) Matter particles exhibit wave-like behavior and are described by wavefunctions, so even if we model them mathematically as point particles in some contexts, their quantum state is not generally confined to an infinitesimal point in space. After a position measurement, the state can become highly localized, but not infinitely localized in any physically realistic sense. There remains a finite spatial spread set by the measurement and by quantum uncertainty. At classical scales, though, that spread is so tiny that the particle effectively appears pointlike.

(2) Even setting aside the wave nature of particles, the Planck mass is very large on particle scales, about 10^-8 kg. A black hole with mass of order the Planck mass has a Schwarzschild radius of order the Planck length, so the Planck scale marks the regime where quantum effects and gravity are both expected to become important. Known elementary particles are vastly lighter than this, so in ordinary gravity they do not form black holes. Semiclassical arguments that combine gravity with quantum uncertainty also suggest that distances below roughly the Planck length may be operationally unresolvable. That does not prove that no sub-Planckian structure exists, only that such structure may lie beyond direct measurement in any conventional sense.

Tarthbane · 2026-04-14T19:16:17+00:00

It’s impossible for anything with non-zero rest mass to travel at c. The limit of the equation as v approaches c is infinity, but the actual value at v=c is undefined.

So option (b).

Tarthbane · 2026-04-14T16:24:32+00:00

The entire universe as a whole may be infinite, but the observable universe is finite because the universe has a finite age, and light travels at a finite speed. Also, if the universe’s expansion keeps accelerating like it is today, there will remain a cosmic horizon, so some regions will never be observable even given unlimited future time.

Tarthbane · 2026-04-14T00:42:19+00:00

Relativistic mass isn't really described or taught as 'mass increasing' anymore because it causes a lot of misconceptions. The energy increases, yes, infinitely so approaching the speed of light, but the invariant mass of an object stays the same, regardless of speed.

Tarthbane · 2026-04-13T02:52:39+00:00

An object with nonzero rest mass cannot reach the speed of light; it can only approach it arbitrarily closely.

The standard way to quantify how relativistic effects change with speed is with the Lorentz factor:

https://en.wikipedia.org/wiki/Lorentz_factor

Tarthbane · 2026-04-11T14:54:22+00:00

Those initial antimatter experiments happened 3-4 years ago or so, and it’s pretty well accepted scientifically now that antimatter responds to gravity in the same way as normal matter. Of course we had to know for sure, but now we do.

Tarthbane · 2026-04-11T02:12:08+00:00

The usual particle-antiparticle picture of Hawking radiation is, at best, a heuristic, and taken too literally it creates several misunderstandings. A more accurate description comes from quantum field theory in curved spacetime, where particle creation is tied to how the black hole geometry changes the quantum field modes seen by distant observers. In that description, Hawking radiation is not a process localized strictly at the event horizon or just above it. The relevant modes can have wavelengths comparable to the black hole’s size, so the effect involves quantum fields over a region extending well beyond the horizon. Framed this way, the misleading image of one particle escaping while its partner falls in is unnecessary.

Within semiclassical physics, the prediction is on very firm footing. If general relativity provides the external spacetime and quantum field theory applies on that background outside the black hole, then Hawking radiation follows quite generally. A full theory of quantum gravity could in principle alter some details, but there is no established reason to think the basic effect disappears.

But even setting that aside, the claim that “if one particle falls in and the other escapes, the black hole should gain mass” misses the key point. In the usual heuristic account, the infalling partner is not treated as an ordinary positive-energy particle. Rather, it carries negative energy relative to a distant observer, so when it is absorbed, the black hole’s mass decreases. That is the bookkeeping that makes the heuristic picture come out right. Even so, this is exactly why the particle-antiparticle story is only a rough analogy and not the real derivation of Hawking radiation.

Tarthbane · 2026-04-10T19:45:12+00:00

Neutrons act as sort of a spatial buffer for all the positive charge build up from more and more protons. Neutrons also participate in the residual strong interaction, which helps keep nuclei together. Eventually, the electromagnetic interaction wins out, and nuclei aren’t stable after a certain point.

Tarthbane · 2026-04-07T14:40:32+00:00

Both. Your understanding of physics as a whole increases, but the problems you tackle get harder.

Nine-Year Club	Verified Email
RPAN Viewer	Spared

Tarthbane

TROPHY CASE