# The beginning/end of the universe

A thought has cross my mind for some time.
What if the universe is expanding, and will start, at a certain point, to collapse (like Fridman's point-of-view).
but, if you look at the space-time as finite but limit-less (as Steven Hawking propose), than all of the physics we know today, will be valid throughout the whole process (big bang -> expanding -> colliding).
Even the Energy-Conservation principle will be valid. so if I'd say that there was a big bang (with X energy), the energy will be converted to velocity immediately after the big-band(sum of Kinetic energy X of all the particles and the gravity). them, the particles will start to expand (the life we know). they will start losing their Kinetic energy over the potential energy (the gravity). there will be a phase shift to collapse (when the gravity will overcome the kinetic energy). then, all the particles will return to their origin - the big Smash). by that, all the energy will return to the first origin and there will be further more big bangs.

this is all true (to my opinion), if there is no other particles at first. also, if there are other phenomenons like other collisions and so, which preserve the general energy/mass and gravity).

can anyone help me foundation my opinion or contradict it?
pardon my English, I'm Israeli.

Gold Member
Dearly Missed
Originally posted by myok
A thought has cross my mind for some time.
What if the universe is expanding, and will start, at a certain point, to collapse (like Fridman's point-of-view).
...
can anyone help me foundation my opinion or contradict it?

Hello myok, even though Fridman was living in Sankt Peterburg/Leningrad and spelled his name "Fridman", many English-speakers know him as "Friedmann"

It is not true that according to Friedmann equations the universe must sometime stop expanding and fall back together again. It depends on parameters in the equations. Putting the most widely accepted values of the parameters into Friedmann's model makes the model predict continued expansion----endless expansion.

The matter that we can see----all the galaxies with all their stars---is only 4 percent of what would be needed to make this expansion get tired and stop and let it begin to contract.

If on top of that 4 percent, the astronomers add their best estimates of invisible matter and energy----the "dark energy" and "dark matter", which they do not know what it is made of----it is STILL not enough to ever stop the expansion.

For dramatic and psychological reasons this could be seen as regrettable---many, perhaps most, people would like to imagine an eventual fiery collapse of the universe, a Big Crunch, leading perhaps to another expansion, a kind of rebirth. This was even considered possible in conventional cosmology at one time and people wrote books about it. But now this view (despite its emotional appeal) has been abandoned. It is not supported by the observations.

Cosmology has changed radically in the past 5 years. The best recent summary of the conventional picture that I know is
by Charles Lineweaver, May 2003.

http://www.arxiv.org/astro-ph/0305179 [Broken]

Cosmology is still based on the Friedmann or Fridman equations
(they are just simplifications of the 1916 Einstein equation) and in them the speed of recession of a distant galaxy does not represent kinetic energy. It is not like a ball thrown up in the air that must eventually lose its kinetic energy (trading it for potential energy) and start to fall back down. This may seem strange, but it is not like the ball----the equations allow it to keep on receding from us forever.

Last edited by a moderator:
Staff Emeritus
Gold Member
Welcome to Physics Forums, myok!

The idea of a cyclic universe (Big Bang - Big Crunch repetition) has been around for a while, and as marcus described would require that the topology of the universe is "closed" (enough matter and energy for gravity to stop the expansion of spacetime). Current evidence indicates that the topology is "flat" and therefore the universe will expand forever (no Big Crunch).

But otherwise, yes, a cyclic universe would seem to play well into ideas about conservation of energy.

Explenation

first, sorry for the mistake in Friedmann's name (pre-apology was added).

I don't understand something.
If the big bang occured, and from then, all the mass/energy released into space is expanding. the only force active there is the gravity between all the particles (which is summed into the "center" of the blast). So that, the speed of the particles will be reduced, until after sufficient (long) time, that the particle will return.
Or not?

Gold Member
Dearly Missed

Originally posted by myok
first, sorry for the mistake in Friedmann's name (pre-apology was added).

I don't understand something.
If the big bang occured, and from then, all the mass/energy released into space is expanding. the only force active there is the gravity between all the particles (which is summed into the "center" of the blast). So that, the speed of the particles will be reduced, until after sufficient (long) time, that the particle will return.
Or not?

I like your spelling Fridman's name the way he would have---but in Cyrillic alphabet (Russian spelling). Maybe we should change!

You should read Fridman's equation sometimes. It is very simple, only four or five symbols, but it says something surprising.

This is why he is famous and why we have his equation. If it just repeated what everybody already knows about the ball going up in the air and falling down then why would we have the equation?

We have the equation because it is carefully derived from Einstein's more complicated equation and because it says something unexpected and puzzling.

The things can continue receding from each other forever.
My wife is calling me to lunch so I cannot explain.

Originally posted by myok
first, sorry for the mistake in Friedmann's name (pre-apology was added).

I don't understand something.
If the big bang occured, and from then, all the mass/energy released into space is expanding. the only force active there is the gravity between all the particles (which is summed into the "center" of the blast). So that, the speed of the particles will be reduced, until after sufficient (long) time, that the particle will return.
Or not?

That is exactly what most cosmologists thought at the beginning of the last decade. Extremely long-range telescopes were used to measure the red shift of objects at great distances, in order to reveal the rate at which these objects were moving away from us and one another in the distant past. Comparing the rate of expansion in the distant past with that which is observed in closer objects (both closer to us and closer to the present), we hoped to measure the rate at which things were slowing down. Turns out, things are not slowing down. Things are not even proceeding at a steady pace. The current rate of expansion of the universe is actually faster than the expansion 10 billion years ago!

This means that some other force must be at work. We have not been able to detect it, we only have theories as to its source, but something is providing energy for acceleration. Either that, or we are completely misinterpreting what the red shift means, in which case the entire Big Bang cosmological model would be in jeopardy.

Still ...

But you can't look at the stars today, and see if all the particles are expanding. they may not blasted as a baloon cover (if they did, all particles need to expand from each other, with the same speed).
Some might blasted faster than other. and then, it is all about gravitation between all the particles. Some can accelerate, some can slow. But in general direction, they are slowing down (the sum of them - shown in "picture").
pardon my sense of art.

#### Attachments

• bigbang.gif
2.1 KB · Views: 469
Staff Emeritus
Gold Member
The Big Bang was not an outward explosion of matter & energy into empty space. It was the beginning of space & time itself. The entirety of the universe was the Big Bang seed (whatever that was...it's unknown) and after the Big Bang, all of the points within the universe got further apart. So, there's no center or edge in 3D space. There is no where in the universe you can point to and say the Big Bang happened there and everything exploded outward from there. In essense, the Big Bang happened at every point in the universe simultaneously.

The trouble with imagining all this is that you cannot view this event from an outside frame of reference. As far as we know, there is no external frame of reference.

Even back at the beginning when the universe was "tiny" it was still boundless in the sense that there was no center or edge (extreme hyperbolic curvature of space)...so perhaps you could reach forward and touch your own back.

The "balloon analogy" (dots on the surface of an expanding balloon) is one way to visualize the expansion of space...but that still gives people the distinct impression of a center and edge, which is not the case (the case would be that the 2D surface of the balloon, which is meant to represent 3D space) is the entirety of existence. The "rising raison bread" analogy is a little better, but still gives the impression of a center and edge. As someone in these forums said...better to imagine an infinite loaf of bread.

Gold Member
The current cosmological model thought most likely is that the universe is unbounded, open, infinite and accelrating (IIRC this is known as a Lemaitre univerese). "How can a universe be infinite and expanding?" Well, one good analogy I have come across is to imagine an infinitely large piece of squared paper with the squares getting bigger.

Gold Member
Dearly Missed
Originally posted by jcsd
The current cosmological model thought most likely is that the universe is unbounded, open, infinite and accelrating (IIRC this is known as a Lemaitre univerese). "How can a universe be infinite and expanding?" Well, one good analogy I have come across is to imagine an infinitely large piece of squared paper with the squares getting bigger.

That image of infinite space expanding is a good one. the square width can represent the scale factor a(t) which is sometimes thought of as a normalized "average distance between galaxies".

I share your perception of the current consensus, at least in essentials. The recent articles I have read support your impression that there is a convergence of opinion among working cosmologists on a basic set of parameters and a "concordance model" of the universe.

they would probably call it the "standard model" if that phrase had not been taken already by the particle physicists.

Instead of "open" what I hear them saying is spatially "flat".
But qualitatively this is no big difference since flat is a limiting case of open.

A representative survey paper by a prominent cosmologist is
Michael Turner's "The New Cosmology"
http://arxiv.org/astro-ph/0202007 [Broken]
another by him is "Making Sense of the New Cosmology"
http://arxiv.org/astro-ph/0202008 [Broken]
There are also understandable and authoritative overviews by
people like Lineweaver (of the COBE group) and Wright (of the WMAP group) and someone the PF poster "Imagine" came up with, Plionis. They all agree on the basic parameters, which is a big change from how it was 10 years ago.

Last edited by a moderator:
ranyart
The interest in analogy to a balloon is expanding stood well in portraying exanding Universe. The problem was nobody realized that at the moment expansion occured, or inflation handed over to expansion and the Universe was set in expanding motion, science neglected to the mechanisim.

For instance, if you take a balloon and attach it to one of those 'pumps' used at parties to blow balloon's up, now the first action inflates the balloon quite rapid, the downward stroke on the pump inflates the balloon with the amount of air contained within the chamber of the pump.

Now, the next action let's air into the pump chamber, but takes none away from the partially inflated ballon, you push downwards, and the balloon inflates a litte again, the radius porportional to the amount of pressure from the air being introduced to the Balloon.

Now this is quite interesting, for if you continue to inflate the balloon thus, you will start to see that each continuous action, introducing air, produces a 'slower' expansion to the balloon, the first stroke inflated at a greater speed than a stroke that was much later, the balloon as it fills, expands in size less?

You can now see that at some moment the surface of the balloon is stretched to a definate limit, each action of the pump produces no movement on the surface of the balloon. But inside the balloon, the pressure is increasing, eventually the balloon will 'pop'.

The observed increase in expansion we see today is from upon the surface of the balloon analogy, because of initial conditions we expect to see a fast expansion, if we were looking from the surface, back to the FIRST ACTION of where the pump pushes outwards the balloon, (and you can see this if you actually inflate an ordinary balloon, the first 'puff' produces an exceedingly fast expansion compared to following 'puffs').

There are many problems using the surface of balloon analogy, one major mistake has been to neglect the early actions are (fast) and when we look from our relative steady 'surface' of within Galaxies, we forget that this surface, if one rewinds the balloon backwards, produces a general flow, that eventually gets much faster, right down to the initial condition.

This itself leaves a number of other paradoxical products within Einsteins fields equations.

For instance, if you are within a surface that is fully extended, and there is unavoidable collapse imminent, then, the big crunch, turns out to be the moment in my above post, where the Universe goes 'pop', there is no gradual de-flation, the stress energy tensor equations show that a big-crunch can happen at the same speed as the big-bang!

The first moment of a big-crunch, is equivelent to the first action of the pump in the inflatory model above.

Gold Member
Yes, your right Marcus, strictly an open universe means one that is infinite, flat and will expand forever, though people today generally seem to use it to mean one that will expand forever.

heusdens
Strictly speaking the Big Bang does not denote the begin of spacetime. The Big Bang theory does not state that.
There are Pre Big Bang theories in development that talk about a possible begin of time, but also that is not strictly speaking The begon of The time, but in fact only of one time component (real time) while another time component (imaginary time) is still beginless.

I think the combination of cosmic inflation with the big bang theory makes much sense, since cosmic inflation solves some problems the big bang theory itself could not solve, it makes observable predictions that can be tested for (i.e. the rimples or fluctuations that caused galaxy and cluster formations, and can be observed in the CMBR), the flatness of space, the homogeneity (distribution of matter throughout the universe), etc.

It seems to me that cosmic inflation already contributed much to the Big bang model, and matches up fairly good with the observations.

And it comes also with the advantage that in a philosophical sense it gets rid of this horrible "begin of time" doctrine, that almost sounds like a theist doctrine (even the pope and Catholic Church once agreed on the Big Bang, since they like the idea of a universe that needed a creator).