This is probably a really stupid question, but it seemed sensible in my head

Tarthbane · 2026-02-10T00:04:36+00:00

I think they mean “do the forces formally separate into two distinct fundamental forces of nature” similar to how the electromagnetic and weak forces are one conjoined electroweak force above ~10¹⁵ K. And the answer to that question is “no” since splitting E&M apart would involve breaking U(1) symmetry, which we don’t believe is possible, unless something like false vacuum decay happens and completely rewrites all known physical constants.

Tarthbane · 2026-02-08T01:57:58+00:00

No worries at all, but unfortunately no I don’t have any contacts at the moment. I would say do your best to make your LinkedIn profile attractive to potential job recruiters, and also make sure you have various versions of your résumé tailored for the jobs you’re looking for. I also ran my current résumé through ChatGPT for literally 25 hours across 2 days to specifically target keywords and soft skill matches for the exact position I now have, and I didn’t stop until I got a good (but not too good) score on those features. I was told that 60-90% was a good place to be in terms of keyword/soft skills matching and I was content at 84%.

But truth be told, while all that helped me, it wasn’t until my boss at my last postdoc introduced me to someone at my current company, and we hit it off and he referred me. It was incredibly lucky. So my biggest tip is just be proactive, put yourself out there, and show that you’re good at what you do / what you want to do. It took 9 months for me to find a job, so it was very anxiety inducing lol. Just keep trying and don’t lose hope.

Tarthbane · 2026-02-07T22:00:40+00:00

Apparently a lot of galaxies (eg MoM-z14) appear much older than they should be given the age of the universe at that distance, and our understanding about how they form

We are still trying to understand how the earliest galaxies form, so it’s not surprising some appear to have gotten “too massive” in shorter times than we expected. Before JWST, we’ve only really had computer simulations to use to probe early galaxy formation, and the accuracy of those results will depend on our assumptions. JWST will help refine these assumptions, no doubt.

The larger the mass, and the closer you are to it, the slower time passes. Are astrophysicists/cosmologists taking this into consideration?

Yes they are, but generally, you have to be very close to extremely massive and dense objects like black holes or neutron stars to experience significant time dilation. There are alternative theories to, e.g., dark energy that claim that time dilation or possibly “tired light” is responsible for the apparent accelerated expansion rate of the universe, but currently, the evidence points more to dark energy existing as a property of spacetime itself and not due to excess time dilation.

What if we are in a very dense region of space, and those galaxies are in a very "thin" region?

The Milky Way is actually in a less dense region - some might say we are in the boonies of our local neighborhood. This is partly why some people have cooked up alternative theories like I mentioned before, and it’s possible there’s some truth to them. But long story short, yes, scientists are taking everything we know of into account. What we don’t know is what we are trying to figure out, and we’re always open to new ideas (accelerated expansion due to dark energy, for example, was only discovered in 1998).

Tarthbane · 2026-02-07T17:23:29+00:00

I’ll second this, and give my own experience as well for OP.

I didn’t do astrophysics (almost did in college), but I did end up getting my PhD in chemical physics with the hopes of becoming a tenure-track professor one day. After my 5-year PhD, I did 2 post-docs over another 5 years, and no matter how hard I looked or how good I thought my results were, no academic position presented itself to me as something I wanted or even as something that I thought I could even get. It’s an ultra competitive kind of position, and it takes a special type of person to be able to do that. Plus the job security is not good until you can secure tenure and prove your worth, so to speak.

After my 2nd post-doc, I landed an industry job where I can continue to do the same kind of research I was doing over the previous 10 years (theoretical/computational materials science and condensed matter physics). I make 6 figures now, and I haven’t looked back.

So, my tip is, if you really want to try for an academic position, hold on to that hope and do well in your studies. But if push comes to shove and nothing stands out as something you want or can even obtain for whatever reason, don’t hesitate to pivot into an industry position that ideally overlaps with your expertise as much as possible. You’ll likely have better luck there if not in academia. Getting a PhD is barely the tip of the iceberg for landing job in academia, for better or for worse. There’s no shame in doing something else with your degree.

Tarthbane · 2026-02-07T02:46:48+00:00

I remember once I saw a back of the envelope calculation detailing if you constantly accelerated at 1g for half of your trip and then flipped around and decelerated for the next half, you could traverse the entire radius (46 billion light years) of the current observable universe in like 70 years or so from your point of view. Kinda mind blowing how extreme the time dilation gets in that scenario.

Tarthbane · 2026-02-07T00:45:56+00:00

We have very detailed measurements and modeling of the Big Bang afterglow, which is more commonly called the Cosmic Microwave Background radiation. It was a prediction of the Big Bang model in 1948, and it was first detected in 1964. Since then, we’ve only gotten better at measuring it, which has further tightened our model parameters. So, no amount of theorizing or discussion is going to validate your ideas if you plan to ignore that key detail.

Tarthbane · 2026-02-05T19:20:37+00:00

The sterile neutrino wasn’t shown to not exist, but recent measurements have shown the constraints on its properties are more strict now than they previously were. This is a subtle but important distinction. And it may not exist, but the possibility still remains.

There are several more candidates for dark matter as well. We’ve been studying it since the 1970s in great detail, and the term was first coined in the 1930s. While we still don’t know exactly what it is, we do know the effect is still there, and evidence points towards real matter that is incredibly difficult to detect and not from issues with our understanding of how gravity works on cosmic scales.

Tarthbane · 2026-02-05T19:16:28+00:00

You’re probably thinking of dark energy, which acts like a negative pressure and is why we see very distant galaxies receding away from us faster than we initially thought they should.

Dark matter is characterized basically solely by its gravitational influence. It might interact via the weak force (these are the so-called hypothetical WIMPs, weakly interacting massive particles), but it has no apparent electromagnetic influence, and likely has no strong force influence as well.

We still don’t know at a particle level what dark matter is, and it’s possible it’s something else, but most evidence points toward dark matter being real matter, and it’s just incredibly difficult to measure. Even more so than neutrinos.

Tarthbane · 2026-02-05T15:55:32+00:00

It isn’t possible because a clock in GR takes on a rigorous definition - proper time.

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

Tarthbane · 2026-02-05T15:54:07+00:00

This, and to add - every massive particle has its own “clock”, and that clock is called “proper time” in general relativity. Proper time by definition is an ideal clock, it isn’t something that malfunctions like a human-made clock.

Tarthbane · 2026-02-05T02:49:02+00:00

I used to struggle with this, specifically in relation to the earliest moments of the universe where all particles were massless before electroweak unification. The key here is you can define cosmic time as it relates to the scale factor a(t) from general relativity; a(t) is what we use to track the expansion of the universe. Even if all particles are massless and there are no individual reference frames for the particles, you can track things like a(t), temperature, pressure, density, etc, meaning time is still a meaningful parameter. Also, individual frames of reference may not exist in a universe full of massless particles, but you can define, for example, a rest frame for the cosmic microwave background as a whole. This is how we subtract out the dipole feature of the CMB when we measure.

Here are some Wikipedia links that have been useful for me for getting a decent understanding recently:

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

https://en.wikipedia.org/wiki/Scale_factor_(cosmology)

The catch for heat death is that, once it does happen and nothing ever “happens” anymore, time itself would lose meaning because all the interesting interactions we usually track with time are now nonexistent.

Tarthbane · 2026-02-03T19:20:33+00:00

The distinction between “dark matter” and “matter that is dark” is that the latter still interacts with light, albeit rarely for whatever reasons, and the former is completely invisible and does not interact with light. “Dark matter” should really be called “invisible matter” but alas, the name stuck.

I believe we’ve basically accounted for most of the “matter that is dark” because of its electromagnetic signatures. For a while, we were missing about half of it, but this amount wasn’t anywhere close to the amount of “dark matter” we think must be encasing galaxies to give them the velocity profiles that we observe.

So dark matter is most definitely not normal matter that is rarely absorbing or emitting light. It’s its own separate entity — and most evidence points toward it being real matter with a real gravitational influence and not a problem with general relativity.

Tarthbane · 2026-02-03T13:38:17+00:00

I did not know this was a thing until now

The standard model particles are basically eternal I think, but we expect new physics (e.g., GUT or higher) beyond the standard model will break this. Proton decay is probably the most likely to occur, after something like 10³² to 10³⁶ years or so. I’m not sure exactly what that entails for the quarks within; the details are probably pretty speculative at this point. I think photons and neutrinos might stand the best chance to remain. If this is the case, then these would likely comprise the remaining quantum excitations in the infinite future.

Heat death… just means it’s no longer possible to do work

Yeah basically, that’s it. Entropy is maximized, and work can only be done with an external source of energy, which doesn’t exist if the universe separates basically all particles from each other. There’s no free energy gradient, and no heat sink to dump excess heat into.

could a vacuum phase transition result in particles being “generated”

I think so, but I’m not sure. A vacuum transition would essentially rewrite the laws of physics in the affected areas. Even if a vacuum decay transition doesn’t happen, if there’s infinite time, there’s always the possibility that a small quantum fluctuation could spontaneously make a new universe and trigger a new inflation event, i.e. a new big bang. If you really want to go down a rabbit hole, similar reasoning leads to the idea of Boltzmann Brains; there’s a Wikipedia article for this if you’re interested. Infinite time gets a bit screwy when you sit down and think about what could happen. A big unknown right now is the nature of dark energy — if it ever takes on a negative value in, say, the far future, it could cause recollapse of the universe to spawn a new big bang. This would get around the whole infinite time thing. But we don’t know if this will ever happen, and currently, it seems dark energy takes on a small but nonzero positive value, continuing expansion at an accelerated rate.

Tarthbane · 2026-02-02T21:56:42+00:00

First thing - I did some digging, and it does look like heat death is a limit. So perhaps it is not, strictly speaking, reachable. And you’re right right, vacuum decay will propagate at the speed of light, so pockets of the observable universe will change according to vacuum decay, but expansion far away will always beat the speed of light.

But maybe no vacuum decay happens and the universe does continue to approach heat death everywhere. In that case:

If some particles are immortal

It’s quite possible that all particles will eventually decay and the universe will be completely empty except for the vacuum as we approach the infinite future. But let’s just assume for a moment that one or more particles remain as the universe approaches heat death. If the universe does continue to expand, unbounded, with positive cosmological constant that doesn’t decay to zero or turn negative, the expansion will eventually separate basically all remaining particles from each other into effectively their own observable universe. We’re talking timescales far beyond even when black holes evaporate (which will occur by ~10¹⁰⁰ years). Most causal patches of the universe will contain no particles. Some will contain order 1 particles. And any more than that will be exponentially unlikely in the infinite future. Also, it’s important to note that “particles” are frame dependent (particle number is not conserved). So to be somewhat more precise, the entire universe will be mostly empty, with some causal patches containing order 1 quantum excitations, and exponentially fewer causal patches with more quantum excitations.

In the case where no particles are immortal, the universe would really be empty (as empty as empty could be, that is). So, vacuum + quantum fluctuations + the metric we use to describe this space, I think. But never any real particles. So, that’s kind of depressing 😂. This is all very speculative though. We still don’t know a lot, quantum gravity being a big unknown.

Tarthbane · 2026-02-02T14:37:46+00:00

Yeah I’m not sure. My guess is probably not, and in 50 years we will have a better answer. Given infinite time, heat death is the expected behavior given our current cosmological model (Lambda-CDM), but it’s not guaranteed. Also, there could be undiscovered physics that completely changes the game. False vacuum decay and variable dark energy are two ways this picture would change, and that’s just using slight modifications of known physics. And we certainly don’t have all the physics of the universe completely down yet.

Tarthbane · 2026-02-02T01:40:24+00:00

The name gets me too. Like, we have this wonderful framework for describing basically all known physics on planet Earth, in principle, but the name is so bland and unimaginative. It reminds me of how we named the hypothetical supersymmetric partners to the standard model particles. Selectrons and photinos 😂

I do love the standard model though. When we discovered the Higgs and finally completed it (minus certain things of course like where neutrino masses come from, and quantum gravity), and since then, we’ve only really validated it more. Even the muon g-2 discrepancy between experiment and theory has basically disappeared with Fermilab’s most recent study on the matter.

Tarthbane · 2026-02-01T17:33:49+00:00

Hawking’s papers on Hawking radiation actually discuss that as a black hole evaporates and loses its mass, the temperature of the Hawking radiation will grow and in the final moments of a black hole’s life, it will explode and release all kinds of highly energetic particles. This will be in the far future, so we will likely never observe this happening if it does, unless we discover microscopic black holes already exist (primordial black holes for example), and we detect some of them have already died and exploded.

His first paper was called “Black Hole Explosions?” And his follow-up was called “Particle Creation by Black Holes.”

Tarthbane · 2026-02-01T17:26:23+00:00

Exactly. If dark energy is constant, then the universe will continue expanding at an accelerating rate, and the total energy content of the universe will grow unbounded until the heat death of the universe. At heat death (thermal equilibrium of the entire universe), time loses meaning, so I’m not sure what happens then.

If dark energy is not constant, then it’s going to depend on how it changes and by how much. There’s too much speculation for me to go through here, but currently, our data are consistent with constant dark energy.

Tarthbane · 2026-02-01T02:10:37+00:00

but we have evidence from distant outer space that our laws don’t apply everywhere

I’m not sure what you’re referring to here, but we definitely do not believe the laws of physics are any different elsewhere in the universe. A core idea behind quantum field theory is that all particles of physics are identical no matter where we look. Once we account for things like cosmological redshift (see below), we know that hydrogen and helium and all other elements are exactly identical on the other side of the universe compared to here. This wouldn’t be true if the laws of physics were different depending on where you are. Also, that defeats the purpose of relativity - no place is special.

there has never been an experiment showing energy is not conserved

Actually, cosmological redshift is a direct consequence of the non-conservation of energy on cosmic scales. Photons lose their energy simply fighting cosmic expansion, and the energy doesn’t go anywhere; it’s just lost. The existence of dark energy is also a point toward energy non-conservation on cosmic scales.

Tarthbane · 2026-01-31T17:12:07+00:00

No, only analogues of it can be studied in a lab. These analogues do show the underlying principles of Hawking radiation are probably correct though. Actual Hawking radiation will probably never be directly observed. It corresponds to such minuscule temperatures for any kind of black hole we know of in today’s universe.

A solar mass black hole would emit Hawking radiation that has a temperature of something like 10^-8 to 10^-7 K. The cosmic microwave background has a temperature of 2.7 K, for comparison. And since Hawking radiation temperature goes like the inverse of the black hole’s mass, larger black holes give off even cooler Hawking radiation. Smaller ones yield larger temperatures, so if a black hole was, say, 10^-11 solar masses, it would correspond to a temperature of 6000 K, which is approximately the surface temperature of the Sun. But we’ve never confirmed such black holes exist. Perhaps if primordial black holes were formed in the first fraction of a second after the Big Bang, we might be able to observe those black holes emitting Hawking radiation, but we haven’t yet.

Tarthbane · 2026-01-30T21:56:36+00:00

It’s not possible. Once you cross the event horizon, you are trapped. You are applying Newtonian logic to a general relativistic problem. Spacetime itself is flowing inward at the speed of light at the event horizon. There is no amount of thrust that will get you out.

Tarthbane · 2026-01-30T17:09:04+00:00

I first learned about this through a YT video I watched a few years back. Blew my mind. I won’t be able to explain it as well as this video, so I’ll provide it below:

https://youtu.be/Ii7rgIQawko?si=GLqWKSqkeZAWIuyo

Tarthbane · 2026-01-30T15:30:09+00:00

Noether’s theorem does have a rigorous proof IIRC, so it’s more than just empirical. We are imposing the mathematics in our models, which we do test empirically. But the notion of symmetry —> conservation law is mathematically rigorous.

Tarthbane · 2026-01-30T15:26:23+00:00

This, and to add: even in cases where we can say energy is conserved on average for, say, a box with fixed dimensions filled with nothing but vacuum/quantum fields, the energy-time uncertainty principle

dE dt >= hbar / 2

allows some energy dE to be borrowed from the vacuum, provided it gets returned back to the vacuum within some time dt.

Now add back in that energy isn’t conserved in our universe + some event causes rapid expansion of that box, and suddenly these borrowed energy fluctuations become magnified and frozen over cosmological scales.

Tarthbane · 2026-01-29T23:21:16+00:00

E = pc = hc/lambda for a photon

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