What do you guys think about this? Is there any way that this could be likely?

Consider a scenario where two planets like Earth and Mars collide, it would break up into smaller bits but they would not merge

But black holes are solid mass left over after a big star collapses Why would this not break when another black hole smashes into it. But instead merge into one?

At the time the CMB radiation was emitted, what was the average density of the universe?

I found one answer on stack exchange that calculates about 5 hydrogen atoms per cubic meter. But wow that seems low, given what the phase transition of the plasma was doing (ie decoupling and recombination).

Help me understand this weird epoch. How would you calculate this?

New to cosmology and trying to learn! I am a little confused. With the Sakharov conditions, there is the requirement that baryon symmetry is violated. Does this occur during quantum fluctuations? What is the relationship between quantum fluctuations and baryon asymmetry?

The big bang expanded things? Yet we see that gravity is an attractive / pulling force, could it be the case that gravity is active at all times, not just in terms of pulling elements towards each other, but also matter towards itself? Say the plabnet getting closer to the sun (analogy) because the sun woudl get denser as it pulled towards itself, higher density = the earth get closer to the sun. The same could happen at an atomic level = the core gets dense and smaller, the particles around it equally get denser and smaller, and they get closer to the core in absolute distance. But because things are relative, they would appear at the same exact distance as before from each other. There ould be less empty space inside the particles, but because things are relative, the core would also be smaller, so the empty space would appear as the same % age as before? This would apply everywhere (gravity) and thus space would appear to be expanding.

I've seen people say

>If everything was shrinking then the distances between everything would be expanding. However, the expansion we see is only between objects that are not gravitationally bound

But if matter was shrinking, its density would increase so things would gravitate proportionally closer to it so that the relative distance would appear to be identical no? I've made a picture to explain why the distance inside gravitationally bound objects would not change inside them but only space between different bound objects.

https://imgur.com/0uPQg9t

It would mean its shrinking and maybe through some way the shrinking might reach a critical threshold and everything being compressed so tightly everywhere that it will "explode" /expand in a big bang fashion all over again?

If the inflationary multiverse model and Einstein-Rosen bridges (wormholes) is to be true, could you theoretically go from universe A to universe B through the use of wormholes?

My eight year old is really interested in astronomy. Specifically, black holes. I was wondering if any of you knew of any good kid's books on the subject. We've listened to several from Neil DeGrasse Tyson. I'm just wondering if there are any other good ones.

I know no cosmic-scale objects in space can avoid the two big forces present. Of course these are intrinsic angular momentum and the other is simple gravity, but the apparent rotation curves seem to be consistently "flat", without tailing off as radius increases.

It seems almost like the inverse square law disappears in this scale, though every component obeys it perfectly well.

So I know we can solve that with a larger and larger component: an invisible sphere of dark matter. Yet it seems impossible to detect in our local solar system and in our particle colliders. Can any other exotic shapes solve this curve with less invisible mass?

If enough mass could stay in a dynamic "double fountain", above and below the galactic disk, wouldn't it create an ideal 1/r gravitational field for a great distance?

EDIT: this is one of the many unexplained edges of CDM as a solution for everything. A rotation curve that stays flat even farther.

https://arxiv.org/abs/2406.09685

I have seen videos of animations of future timelines of infinite trillions years later where overall infinite will end and it still makes me sad, but Is there a possibility that the universe could 'reboot' after it's death or somehow scientists in thousands or millions years later will save it ?

I intend to keep this very short and straightforward.

Could the Penrose diagram of the multiverse also be connected to the many-worlds interpretation of multiverses, or would they be entirely separate from each other and have no correlation? (If both are true)

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Due to the nature of the Boltzmann equation it seems like we can’t make the nice assumption of being in the CM/lab frame. I attempted to find an expression for dtheta/dOmega but my expression is quite ugly and I am assuming there is an easier way to go about it. If anyone has any references on how to approach this in a more efficient manner I would greatly appreciate it!

How would you go about trying to build a universe from scratch?

I made an attempt at it and put it into video form. At this point I know it's full of flaws. But I am wondering how smarter people might do it.

https://youtu.be/q3yFcDxsX40?si=pN9eukLZf-njpGZE

Hi everyone,

I’ve been thinking about dark energy and the accelerating expansion of the universe, but I’ve quickly hit the limits of my knowledge. I wanted to run this idea past the experts here to see if it’s worth exploring or if I’m just misunderstanding the physics.

Here’s the gist:

The accelerating expansion of the universe suggests an ongoing addition of dark energy to "fuel" this process. However, this seems counterintuitive to me from a thermodynamic perspective—it feels like an energy "pump" that somehow keeps growing without any clear mechanism.

What if, instead, cosmic expansion isn’t about adding energy but is actually a mechanism for dissipating energy? In this view, expansion would allow light and particles to "age" and lose energy over time, which could naturally explain phenomena like redshift. The uniformity of this process might also explain why it doesn’t clump like dark matter and why the rate of expansion appears so perfectly balanced—it’s not coincidental, it’s inherently self-regulating because its role is to dissipate energy.

This perspective might not change the math (it would still align with Lambda in general relativity), but thinking of dark energy as a dissipation mechanism rather than additive energy seems conceptually different. It also feels less like a perpetual motion machine and more like a thermodynamic process.

So my questions are:

1. Is this idea fundamentally flawed based on what we already know?

2. How might this interpretation manifest differently in predictions or observations?

3. Could this hypothesis be tested in any meaningful way?

Thanks in advance for any insights—or for pointing out where I’m going wrong!

Edit: for the people asking where is the maths. I'm actually not proposing a change to the maths. We have the cosmological constant lambda as part OF general relativity (GR) and we've given it a slightly more positive value to account for the observed expansion.

The Dark Energy interpretation of this doesn't make any strong claims that the energy is necessarily uniform everywhere, though it does seem to be everywhere we observe, it also doesn't say that we'd expect the rate of expansion to necessarily hold constant.

With the energy dissipation interpretation I’m exploring, we’d strongly expect uniformity—aligning better with the idea of a single constant. While it’s conceivable the constant could change over time, this interpretation suggests it would evolve in one direction and be fundamentally tied to the universe’s properties, rather than existing as a fully independent dimension.

This interpretation also sets tighter parameters on what we might observe compared to the dark energy framework, which doesn’t make as many specific claims about uniformity or the constancy of the expansion rate. However, I’m not sure whether it leads to any testable predictions or if it contradicts existing evidence—hence, I’m throwing the idea out here to see if it sparks discussion or insights.

I'm a layperson so really wasn't sure how to title my post so as to best communicate my question, but hopefully i can nuance a bit here; I'll spare you the long backstory and just get into the question.

My question is this, we often hear that various fundamental values, constants, etc. could be between x and y; but what actually determines that? and how much could that vary while holding other aspects of the early universe relatively fixed?

For instance i was reading something about respective masses and lifetimes of the bottom quark, charm quark, tau lepton, and higgs boson being ideal for observation; falling between the optimal 0 and 2 GeV of a possible 0 - 150 GeV (in the case of the mass of the charm quark for instance). But what would have had to have been different about the early universe for these masses (and by extension their lifetimes) to have actually been different? Could these values be significantly different even if other aspects of the early universe are held fixed (strong nuclear force, cosmological constant, speed of light, quantum stuff etc.; idk, like i said im a layperson)?

EDIT: i just noticed i also included "frequencies" in the title, that's bc i was also going to ask about something else but ultimately decided to keep the question more focused.

If we look at all objects from a molecule which is comprised of an atom and electrons where the electrons are rotating around the atom bounded by gravity to planets rotating around a star also bound by gravity to stars rotating around the supermassive black hole around the center of the galaxy also bound by gravity.

Given the above is it logical to infer the possibility that galaxies themselves are gravitationally bound to some ultra supermassive objects whose scale is simply too large for us to contextually see? If there are numerous ultra super massive objects, could this theoretically explain why from our perspective everything appears to be moving away from us.

Recently by a coincidence I have found this book by Katie Mack and it really shines. 😁 Do you know and can suggest anything similiar on astrophysics/cosmology for further reading?

Inspired by the recent post about "why do black holes exist", I started reading about them more and apparently at the center of every large galaxies there is supermass8v3 black hole. I tried to read more as to why, but the text was too dense. Can someone explain like you are explaining to the child what are the leading theory/guess as to why they are there?

A classic conundrum is asking what happens to the energy of redshifted light. Intuitively, one would guess that the equation for energy would be E = (hc/w)*(1+z) where h is Plank's constant, c is the speed of light, w is the observed wavelength, and z is the redshift. The published equation doesn't have the (1+z) factor though.

While trying to research it, I'm not even sure if introducing that (1+z) term would represent a violation of relativity. As far as I can tell, the reason this equation doesn't violate conservation of energy is (waving hands) spacetime curvature.

I would have a much easier time accepting the Plank relationship for the energy of a redshifted photon if I could find a paper that describes an experiment where the researchers measure the energy of a redshifted photon. However, I can't find any such study. It doesn't seem like performing such an experiment would be too difficult... A CCD camera effectively counts photons, so if we could use some bolometric device that responds to total energy levels, it would be straight forward to check the validity of the Plank relation.

If there aren't studies that have done this, would it be feasible to do this experiment using backyard telescope equipment?

The most basic physics that i know is that if an object has bigger mass than other objects, the object surrounding will revolve around it. Universe has galaxies which can move, but it doesn’t move to one centre. Ideally black holes can be a centre of universe. I don’t know can black hole be a centre of universe.

Inside a black hole, I understand that you can't really move in space but that you move only towards the singularity. Is this somewhat accurate?

So this thing, does it exist in space at all? What is inevitable, or why that choice of words? Does it exist in our time, for the rest of time? I don't understand the context of what is being conveyed. I feel like I lack the understanding to express what I'm asking in a way that is meaningful here

Sorry for the incredibly broad question, I just like to pick the brains of people with more knowledge and schooling about this stuff than me: I understand the consternation over the very real possibility that once anything affected by the curvature of spacetime - everything, basically - passes through a black hole's event horizon, all the information that "thing" contained is lost to us forever. Even after the trillion plus years it would take for Hawking radition to "evaporate" the BH. Although the "lost" part is obviously very interesting (and alarming) to me, I want to know what you folks think about the "information" part. Is it physically real, or is it just a construct to try to explain something we don't have a good grasp of? I mean, there's no bit, or "informaton" in the Standard Model, last time I checked.

New to this field. Why do black holes even exist? I'm not asking what they're made of or how they work— I mean, why are they even a thing in our universe? What about the laws of physics and the way the cosmos is structured leads to something as extreme as a black hole coming into existence?

Thanks!

Is our solar system moving because of kinetic energy that came from an explosion at some point, or are we pulled by gravity into the direction of a black hole? Or is it something else?

Do we humans have some idea what we’re going towards at the moment? I hope it’s not something dangerous 🥸

Thanks !

A number of articles have claimed that the recent DESI results rule out alternative theories of gravity that might explain the apparent effects of dark energy. I'm guessing they are referring to alternative models like MOND.

However I haven't been able to find an explanation of exactly what data/findings lead to this conclusion.

I'm wondering if anyone here can provide a modestly technical explanation for the informed layperson?

Here's one example article:

"... DESI not only confirms Einstein’s theory but also rules out alternative models of modified gravity that attempt to explain the accelerating expansion of the universe."

https://dailygalaxy.com/2024/11/desis-mind-blowing-breakthrough-validates-einsteins-theory-and-reveals-the-mysteries-of-dark-energy/

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In regards to the 1st second of the big bang timeline, there seems to be 2 different and contradictory cosmology models which is confusing.

1. Inflationary Model

cosmic inflation --> "hot" big bang

A period of cosmic inflation is followed by a "hot" big bang

Inflation lasts an unknown but minimum length of 10^-32 seconds

In the start of the big bang timeline, time t=0 is the final fraction of a second of cosmic inflation.

https://bigthink.com/starts-with-a-bang/ask-ethan-cosmic-inflation-big-bang/

2. Traditional/Standard/LCDM Model

"singularity" big bang --> cosmic inflation

A "singularity" big bang, a "single originating event", is followed by a period of cosmic inflation.

Inflation lasts a maximum of 10^-32 seconds

In the start of the big bang timeline, time t=0 is when the big bang singularity occurs.

There is a series of "epochs": Plank -> Inflation -> Electroweak -> etc

https://en.wikipedia.org/wiki/Chronology_of_the_universe#The_very_early_universe https://en.wikipedia.org/wiki/Lambda-CDM_model

Have I summarized these 2 models correctly? Am I wrong in thinking the traditional/standard model is an obsolete model? Most people agree that cosmic inflation came before the big bang right? And most people agree that inflation lasted an unknown length? Because once you accept that, the traditional/standard model that starts with a big bang "singularity" doesn't make much sense.

If inflation lasts an unknown length of time it could have lasted 10 billion years. In which case it would have started 10 billion years before t=0 in the big bang timeline. So it seems senseless to stick a "big bang singularity" creation event before inflation in the timeline that might start 10 billion years before the timeline starts. Time t=0 is still the earliest time we could extrapolate backwards too so there would be no way to know what might have happened 10 billion years earlier. Also, such a singularity wouldn't seem to be related to the rest of the big bang or the timeline.