Universe Definition and 1000 Threads

If the universe is very large relative to the observable universe and it is spherical, and the observable universe is well away from the outside region of the sphere, more towards the center, is spacetime approaching flat for the observable universe? I always assumed the answer is yes, but then...

I just heard that the universe inflated from the size of an atom to the size of our solar system in 100 seconds. Wouldn't that exceed the speed of light?

Let me quote Stephen Hawking who sadly just passed away, in his book A Brief History of Time: The total energy of the universe is exactly zero. The matter in the universe is made out of positive energy. However, the matter is all attracting itself by gravity. Two pieces of matter that are...

The hyperboloid with equation: ## z^2=x^2+y^2-1 ##, embedded in standard 3-D Minkowski space ( +, +, - ) so that ## z^2 ## is negative, has positive Gaussian curvature equal to 1 ( as found directly from its metric: ## ds^2 = \sqrt{ -dτ^2+(Coshτ)^2 dθ^2 } ## induced from the ambient Minkowski...

Hey. I was just wondering what people mean when they say physics and the universe is computable, I always thought it meant that the universe is measurable/calculable, but someone said to me that it meant the universe works like a computer/simulation. So, what does it actually mean?

Is there empirical evidence demonstrating that the entire universe and the known (directly) observable universe are one and the same? Is there definitive evidence that what we observe (directly) is simply not a small patch of something much larger? That instead of our observations...

Good morning. The question of the "continuous" or "discrete" nature of the universe is the subject of diatribe among the greatest physicists in the world. I would like to discuss the same topic, but asking a question about the aspect of continuum in classical mechanics. The use of mathematical...

According to what I've been taught, the distance-related redshift seen by Edwin Hubble is an artifact of an expanding Universe. That is, as light travels through space, space itself expands, redshifting the light (matter is embedded in space and does not itself expand but is instead carried...

If the universe keeps expanding and eventually ends in a "big freeze" or heat death, does this contradict the third law of thermodynamics? The third law of thermodynamics states that a crystal at absolute zero has zero entropy. Since the entropy of the universe can never decrease, as the age...

Now, we all know all physical things are related to math. EVERYTHING is related to math. Now, we have all heard the theory that we are all living in a simulation, right? What if... math is the coding of the simulation and throughout time we have been decoding our own simulation.

The universe seems to be expanding since the farther away an object is, the faster it is moving. However, because of the finite speed of light, the farther away we look in distance, the further back in time we look. Does that mean that galaxies were moving faster in the past and are now slowing...

If the universe was in an energy eigenstate then d<A>/dt = 0 for any dynamic variable A. Stuff moves which implies that the Universe isn't in an eigenstate. What factors drive the energy spread?

If a big bang could occur at a point in space-time, couldn't we at least speculate that many big bangs could have occurred at many other points in space time, and could have resulted in many parrelel universes evolving far away or in close neighborhood of our own one? And then, could it not be...

Penrose's CCC model posits a mapping between our big bang and a future one. This is based on the idea than in the far future there will be no mass and at the big bang there was no mass. So in both cases the universe looses track of scale. I am aware that the idea of there being no mass in the...

Hello. I've been doing some reading about Friedmann's three types of GR solution that yield closed, open and flat universes. I think I can grasp the closed solution best. As far as I understand it a closed universe is finite in both time and space. It begins, grows and then collapses in upon...

I don't know this question makes sense or not but; Respect to the two possible size universe models (finite and infinite) how would be the evolution of the universe without the cosmic inflation?

What is the dark energy which apparently is causing the expansion of the universe to accelerate?

As we know the universe is expanding. Could this accelerating expansion contribute or cause black hole evaporation given that the strength of the gravitational force does not depend directly on time, while the distance of two given points in space increases with time? Sorry if my approach is...

After reading the wikipedia article and looking at many other threads on this forum I am still having a hard time understanding the difference between the Observable universe and the entire universe... Why is the entire universe not observable to us? The Big Bang happened 13.8 billion years...

In a holographic universe model, could our 3D universe be encoded in 3D and still be a holographic universe, instead of 3D information encoded in 2D space? Or is the standard model (non-holographic) of the universe already 3d information encoded into 3d space...

Greg Bernhardt submitted a new PF Insights post Intro to Big Bang and Infinity Concepts Continue reading the Original PF Insights Post.

If we live in a multiverse, are our equations about this universe valid in all universes?

There are couple things that keep me questioning about the nature of the universe. Let me start from the begining. Big Bang happened and our universe was created, and from now on let us suppose that the universe is infinite in size. Later on, the universe expands and after a time we can see...

How can the universe keep on expanding if it's infinite? Expanding metal, like a cube of aluminium, has a surface area which forms a border for the matter contained inside. So the universe must have a border for the matter it contains.

Is there a possibility that we create a wormhole sometime and switch between universes? First of all what is required to make a wormhole? What will happen if we made a wormhole? What proves that there are many universes and that we can switch between them??

Greg Bernhardt submitted a new PF Insights post This article is part of our student writer series. The writer Arman777, is an undergraduate physics student at METU A Journey Into the Cosmos - FLRW Metric and The Friedmann Equation Continue reading the Original PF Insights Post.

So recently I saw a video with Richard Gott explaining a self-creating universe. There's three quick questions I'm confused about that I did not understand very well from the video: 1. How does a closed time-like curve allow the universe to self-create? Wouldn't the universe just go around the...

Dear All I know that my question is not so related to this, but i will appreciate any answer. Why could we have 2 different models for the universe ( one expanding, collapsing...) although both are receiving the same experimental data... I need to write an essay about this topic. I have ideas...

What if there is a universe with just one point charge in existence....just a charge with vast emptiness around it....now here all laws of physics remain same... But is the concept of field and energy defined there??

Greg Bernhardt submitted a new PF Insights post Coordinate Dependent Statements in an Expanding Universe Continue reading the Original PF Insights Post.

would that mean the Big Bang never happened? If not, please explain to the best of your ability as to why. Much appreciated.

Hi, I am new here. A while back I read articles about observations of supermassive black holes in the early age of the universe. What are the hypotheses that would explain how these black holes grew so large and so fast so soon? Could inflation play a role here? Like how inflation would be able...

This may be a silly question, but how do we give a shape to the universe if it is The same from every point we look?

It may sound stupid but something bothers me and I want to ask This question come to my mind due to another thread, https://www.physicsforums.com/threads/entropy-and-derivations-is-my-logic-faulty.935533/page-2#post-5910474 In Friedmann Equation we are assuming that universe is homogeneous and...

Is it expanding from the edges? Or from the center?

Greg Bernhardt submitted a new PF Insights post A Journey Into the Cosmos - The Friedmann Equation Continue reading the Original PF Insights Post.

I'd like to know people's views on the possibility of Black Holes being 'Gateways' or 'Portals' to parallel universes.

Hello. I am starting to learn about thermodynamics. (i'm going to use lower case "d" for delta) Energy is neither created nor destroyed. So dU = 0 for the universe as a whole. If the universe is constantly expanding, then it must be doing work on the vacuum around it, right? So W is a...

Hi, I am quite confused about followed question, I think scientist think the last scattering surface was dense plasma at the temperature of 3000K. If the today's universe much cooler and less dense then "the last scattering surface" how can anyone says entropy increased by time? Isn't universe...

If the universe suffered a false vacuum decay, would this change physical laws? Could it change the universe so much that it would allow our universe to be a multiverse of level 1, 2, 3 and 4? https://en.wikipedia.org/wiki/Multiverse

This has been a real famous analogy and I understand it, except the fact that the balloon surface is a 2D structure. How is it possible to depict a 3D universe on a 2D plane ? What happens when we work with stars at multiple planes ?

This may sound like a noob question but please help me out guys.Will universal acceleration ever retard ? And if it continues to accelerate for all eternity, where is it getting this never ending energy from ?

I am trying to read through this paper discussing what quantum fluctuations mean in their various contexts, particularly in de Sitter space. I have come across this passage and am curious to what it actually means? https://arxiv.org/pdf/1405.0298.pdf pg. 10, second paragraph: "If a quantum...

Does the presence of the cosmological constant modify the rate of expansion of the universe even during the earlier deceleratingly expanding phase of the universe?

Normally when one is asked what is the universe expanding into, the answer is its not expanding into anything. Space itself can expand. However In the context of eternal inflation, our universe is a bubble in a sea of inflating space. And hence assuming eternal inflation is right, does the...

I understand that there are roughly ##N=10^{79}## nucleons in the visible Universe. This number comes from adding up the nucleons of ##100## billion stars in ##100## billion galaxies in the visible Universe i.e. $$N=\frac{10^{30}}{10^{-27}}. 10^{11}.10^{11}=10^{79}$$ where mass of sun is...

I have a question regarding the process of getting towards equilibrium in our universe. If we imagine a causal patch with our planet at the centre, every planet will redshift away from us an after a while the planet itself will disintegrate, let's call this process the decay of Earth. Eventually...

It explains very well many aspects of the Universe. Why should there be any angular momentum though? The idea that the Universe is itself intrisically rotating doesn't make sense.

Some theorists have put forth the idea of an atemporal universe - ie. a universe without time - claiming that time is an illusion or abstraction created as a placeholder for what is really entropy. https://phys.org/news/2011-04-scientists-spacetime-dimension.html So they're claiming that...

Is knot theory taken seriously by the sconce community , This seems to be a novel theory which explains why our world is three dimensional. https://www.sciencedaily.com/releases/2017/10/171016190308.htm An international team of physicists has developed an out-of-the-box theory which proposes...