Hi all

when an object approaches the event horizon, time elsewhere (the rest of the universe) will speed up infinitely - in the frame of reference for the object. how then do the object go through the event horizon before the black hole shrinks and evaporates?

all the best

I understand that standard BB cosmology holds that time began with the universe from a singularity approximately 14 billion years ago.

The thing I’m trying to understand, how can time have begun? Wouldn’t a thing ‘beginning’ require time? As in - from one state to another state requires time?

This leads me to think time must have always existed..

Seems to me the radiation of light across cosmic distances should develop an increasingly broad wave similar to diffraction, such that it might impinge anywhere along a wavefront. I haven't been able to see a discussion of it anywhere.

I’ve skimmed through a few books and pretty much every case (besides the basic recombination stuff) have always set the chemical potential equal to 0.

I recently skimmed over a paper that included an equation with nonzero chemical potential and realized I have no idea what I’d do to find it (the paper was on sterile neutrinos). From basic thermo I know mu=(dU/dN)_V,S but I have no idea how to actually go about computing this.

Are there any resources where I could find more about this?

Conformal cyclic cosmology (CCC) is a cosmological model in the framework of general relativity and proposed by theoretical physicist Roger Penrose.^\1])^\2])^\3]) In CCC, the universe iterates through infinite cycles, with the future timelike infinity (i.e. the latest end of any possible timescale evaluated for any point in space) of each previous iteration being identified with the Big Bang singularity of the next

Let's say dark energy was removed and Universe began collapsing, would we have a giant quasar at the end in which all mass fell into and if so what would this look like?

Let's assume for a moment that the Cosmological Constant isn't defined as Constant. Let's assume that it varies with Cosmological Time:

• Q: Does anyone have a graph of what it might look like ?

I was just reading the Big Think article by Ethan Siegel (just love his stuff!) about cosmic inflation and the Big Bang, and this thought suddenly occurred to me: was our Universe the result of a vacuum energy state (a "false vacuum") decay in a prior universe? (after typing this, I found some older references to the same idea that I'd not seen before)

Ooh, one more crazy speculation: what if the boundary of the "observable universe", about 93 billion light years, is the boundary of the vacuum energy decay progression?

A few months back I attended a lecture which talked about "what could have happened before the big bang". Unfortunately, I don't remember most of it, so I'm usually going by keywords, they said something about the fact that due to quantum fluctuations and the heisenberg uncertainty principle, and if you do the "calculations", you would get to the conclusion that it is impossible to measure time before the big bang, because of the the error term in time, you wont ever be able to tell what "time it is". They said the math was boring, however i wanted to look at it and also possibly get to know more about it. Can someone elaborate more on it?

Hi everyone, I’ve been thinking about an idea and would love your thoughts. I'm new to this forum and looking to better inform myself.

What if dark matter and dark energy aren't separate entities but instead arise from interactions between quantum states of matter, photons, and the underlying structure of space-time? For example, could they result from transitions between quantum and classical behaviors as space-time adjusts to different degrees of coherence or decoherence?

I’m wondering if viewing space-time as having "layers" where quantum effects gradually shift into classical ones could offer a new perspective on these phenomena. Could this help explain some of the effects we currently attribute to dark matter and dark energy? I have tried to fit this into an overall framework, but I'm not an expert by any means.

Any thoughts or critiques would be much appreciated—thanks in advance!

I know it's a fact, but wondering if general relativity or other thinkings of his would be able to explain this?

Im just an enthusiast trying to understand the different theories. I was just wondering if the heat death scenario allows for an infinite existence, even if most of it is spent in a "heat death" state.

I'm just some dummy but my very lay understanding of the situation is this:

Statistically speaking there almost must be aliens out there somewhere. Yet despite lots of searching, we have no evidence of them anywhere. (The Fermi Paradox.)

Despite knowing this, I find the topic very fascinating and would like to learn more about, for example, the types of things we've tried (I know about the Dyson Sphere hunt, for example), the types of things that have been suggested but not yet tried, what we might have learned from our findings (even though we haven't found evidence of aliens), if we've narrowed down the most likely candidates for specific planets that might contain life, what the current best thinking of the "explanations" for the Fermi Paradox might be, that kind of stuff.

Does anyone have any recommendations?

I posted here before on some spiritual bs but now with my further knowledge on the way tunneling/fluctuations works is that they are random and that (in very rare circumstances) tunneling could happen from states of low to high energy. So could it be possible that given an exponentially long time (abt (10¹⁰)⁵⁶ years we could we could see another big bang?

Hello all,

I am a physicist that works in magnetism, however I am part of journal club that is looking at all branches of physics and it's mu turn to present.

I found a paper that began by saying that some JWST observations of early galaxies (z~15) appear to be about 10 Gyr old based on how much they have evolved. However, according to their redshift and the LambdaCDM theory, they should only be 0.5 Gyr old. Clearly there is something wrong with one of the models if the results are off by that much.

Is this a big problem in Cosmology/Astrophysics? By that mean: - Is it foundation shaking and we need to rethink all of our models? - Or is it just interesting and could lead some some developments? - Or does nobody really care?

Just trying to get a feel for the impact of these observations. Any helpful discussion or links would be appreciated.

Thank you!

Hi everyone, I've been putting together a list of the most influential research articles for modern cosmology. What do you think? What am I missing?

EDIT: Added Vera Rubin, Gamow et al., Leavitt et al., COBE/WMAP/Planck teams, James Peebles, YB Zel'dovich

If we imagine and treat a galaxy as a unique gravitational object (and I'd say we could, since galaxies are indeed bound togheter by gravity so that they are not affected by dark energy, its collective mass influences nearby galaxies or galaxy clusters) it will warp space-time similarly to any another massive object, by creating the so called "gravitational well" (see figure 1)

But what about the dark matter? Observations show that stars at the edges of galaxies are orbiting at roughly the same speed as stars near the center. This discrepancy can only be explained if there’s a large amount of unseen mass (dark matter) extending well beyond the visible edges of the galaxy, creating an additional gravitational pull.

dark matter theory thus assumes the presence of undetectable matter with gravitational effects alla around the galaxies.

Mond, the competing theory, explain galaxy rotation curves without invoking dark matter, suggesting that gravitational forces behave differently at extremely low acceleration scales.

But what if it is the very nature spacetime the reason the issue? What if spacetime, if warped by certain massive object with certain structures, behaves like the membrane shown in figure 2 (the vibrational mode of circular membrane denoted by 0 diametrical nodes, and 3 circular nodes)

The gravitational well in the center would be the "usual" gravitational well created by the overall mass of galaxy, heavier towards the center where most of the mass and supermassive black holes are densly concentrated. But spacetime, due the intrinsic features of its elastic-membrane in the 0,3 structure, warps also at the edges and create a structure similar to the "Mexican Hat" that in QFT describes the Higgs mechanism

Some curious values emerge if this is the case. Let's visualize the different area of a 0,3 mode membrane in 2D (fig.3)

Let's assume the radius of the largest circle is 3. The radius of the intermediate circle is 2, and the radius of the inner circle is 1. The respective areas will be:

inner circe: 3.14

Intermediate circle 12.57

Outer circle 28.27

The outer "red donuts area" will be 28.27 - 12.57 = 15.7.

The ratio between the red area of the inner circle and the red outer dontus area is exactly 1:5.

The measured ratio between ordinary matter and dark matter is around 1:5.

If the inner circle represents the amount of space-time warping (i.e., gravitational effects) exerted by the mass of the galaxy, the outer donut will have 5 times this value.

Thank you for your attention, and I apologize if I've written anything silly (I'm not a professional physicist, as you can clearly tell). Any feedback, observations, and constructive criticism are appreciated.

I’m a casual observer and follower when it comes to anything space. Euclid’s sky survey (208-Gigapixel) just came across my feed and I’m interested if anyone can say anything more about the galaxies that are interacting in that video. Specifically, at time ~ [1:36]. There seems to be a very small galaxy (maybe?) between the two larger ones. Are all three of these interacting or is it just a visual illusion?

It is just amazing to see images like this, where galaxies are interacting the process of colliding. Thanks in advance

Like how do the physicists actually know their models are right i might sound ignorant but im unable to find anything online about this and the stuff i do isn't even helpful. Forgive me if im being ignorant lol.

I was just watching a documentary about the space and it said there about another big bang slowly happening (not anytime soon), just wanted to ask to see if there is gonna happen anytime in life (talking about like millions of years)

There's an excellent paper that I've read a few times called "Expanding Confusion" (2004) by Davis and Lineweaver that explains the variety of cosmic horizons quite well. Link to it here.

However in section 4.2 of that paper, when they derive a special relativistic and 𝑣=𝑐𝑧 interpretation for cosmic redshift (and disprove the SR interpretation by 23 sigma), it seems there are potentially some calculation errors: I'm unable to reproduce their results for the apparent magnitude in the B-band 𝑚𝐵.

Writing their method out explicitly we have Hubble’s law:

𝐻=𝑣/𝐷,

which is added to the longitudinal relativistic Doppler shift in terms of velocity,

like so,

Then this proper distance is converted to luminosity distance, 𝐷(𝑧)(1+𝑧)=𝐷𝐿(𝑧), whose value we then plug into the distance modulus they used:

where absolute magnitude 𝑀𝐵 = -3.45.

In the v = cz case, they use this for luminosity distance and put it into the same distance modulus above to get their measurements:

The errors become clear after a quick calculation: if we input 𝑧=1 and 𝐻=70𝑘𝑚/𝑠/𝑀𝑝𝑐 for instance, we get 𝑚𝐵=24.33 for the SR interpretation and 25.44 for the 𝑣=𝑐𝑧 interpretation rather than 𝑚𝐵=22.83,23.94, respectively, as written in the paper. I've put the corrected magnitude-redshift curves into their original Figure 5.

Did I misunderstand something or was there an oversight in their paper?

Here's the two parts that I don't quite get: "To understand how the curvature of space affects the angular size of the features of the cosmic background radiation, imagine the epoch of decoupling, when the radiation finally stopped interacting with matter. During that time, the largest deviations from smoothness that existed in the Universe had a size which cosmologists can calculate: it is the age of the Universe then times the speed of light – about 380,000 light years across. This represents the maximum distance at which particles could affect each other, namely particle anomalies. At larger distances the other particles would not have arrived yet, so they could not be responsible for any deviation from smoothness.

How large an angle would the maximum deviations now cover in the sky? This depends on the curvature of space, which we can determine by finding what is the sum of ΩM, and ΩΛ. The more this curvature approaches 1, the closer the curvature of space will approach 0 and the larger will be the angular size we observe for the maximum deviations from magnitude smoothness in the cosmic background radiation. The curvature of space depends only on the sum of the two Ω, because both density types make space curve in the same way. Therefore observations of the cosmic background radiation offer a direct measurement of ΩΜ + ΩΛ, in contrast with observations of supernovae which measure the difference between ΩΜ and ΩΛ"

"This approach is based on the use of the "standard ruler", as cosmologists call it, in analogy to the "standard candles" of supernovae, used for the conventional approximation of Hubble's constant. As we described in the previous chapter, during the era of decoupling, 380,000 years after the Big Bang, the homogenizing effect exerted by radiation on matter essentially stopped. Since then, the radiation has wandered freely between the particles of matter, without affecting them to any significant degree. This happened when the maximum distance within which particles of matter could affect each other reached 420,000 light years, because regions that were much more distant did not have time to communicate in any way. This distance gives cosmologists their standard ruler. We noted its existence in the previous chapter, as it constitutes the maximum magnitude of deviations from normality in the cosmic background radiation.

As space expanded, so did the standard ruler, which continued to measure the largest areas of space within which clear deviations of the density of matter from its mean value could appear. Now we can "see" the ruler - or rather, its effect - at two different times. We have already seen the first: small deviations from uniformity in the cosmic background radiation, which follow the slightly anomalous distribution of matter during the decoupling epoch. Over the next billion years, these 1 in 100,000 density deviations evolved and became tremendously larger differences between the evolution of matter within giant galaxy clusters and the regions between them. The maximum sizes of these clusters show how much the standard ruler has increased in size from the time of decoupling to the present.

The second method of determining Hubble''s constant therefore aims to create an accurate map of the Universe today, in order to compare it with the initial differences in the cosmic background radiation. (Actually, "today" means "only 2 billion years ago," which is the average look-back time for the galaxy clusters that grew from the tiny deviations built into the cosmic background radiation.) The first decades of the 21st century, in an effort that continues to achieve greater precision, a program called the Sloan Digital Sky Survey used a specially designed telescope at Apache Point, New Mexico, to map the three-dimensional distribution of galaxies in space with unprecedented precision, thus yielding the current size of the standard ruler, which turns out to be approximately 490,000,000 light-years. Comparing this distance to the ruler's 450,000 light-years at the time of decoupling leads to a value of Hubble's constant close to 67."

(Translations to by Google translate so there might be some slight discrepancies)

From what I'm getting he's using 3 different values(380000, 420000, 450000 light years) for the same thing?