I want to know the exact numbers and preferably the calculations that gives the size of the universe at the Big Bang and the initial rate of expansion at the moment of big bang. Does somebody know this? thanks!
67216
Conceivably it expands with the speed of light. I do not know, but curious, if there is an answer. At what velocity, does our universe expand?
31721
At the time the Big Bang happened the matter had enormous density. According the GR (I may be wrong here) such density dilates time.  If so, could it be that the time periods just after Big Bang which are usually considered happening in small part of a second (such as the Planck epoch), in reaity took billons of year (or may be, infinity) but due to time dilation appear to us as spanning only microscopic parts of a second? Could it be that the age of the universe is dramatically underestimated?
16966
I have read about cosmological constant given by einstein in universe in nutshell as well as in general and special relativity. But still I am not able to understand the aim to use it or to introduce it. Can someone explain in brief.
99136
Am I right in understanding that current theory states that Big Bang originated from a single point of singularity?  If so, would this mean that this was a uniform point?  If so, as the universe expanded, what factors contributed to transforming its expansion in a non-uniform manner, to the point of what we can now observe? E.g., non uniform densities of matter and non-uniform cosmic microwave background noise, the latter we are told being the imprint of the very early universe.  I'm making assumptions here but I would be interested in understanding how, if it started uniformly, what made the universe become diverse.
54796
The theory of Inflation explains the apparent consistency of the universe by proposing that  the early universe grew exponentially for a 1E-36 seconds. Isn't a simpler explanation that the universe is just older and so the homogeneousness comes from a slower more steady growth? Is there any evidence that rules out a slow growing universe and supports Inflation theory?
50119
I saw  hosted by Stephen Hawkins.  And if I didn't get it wrong, it says that there was no time before the big bang, time was created there.  So how can anything happen when there is no time (eg: the creation of time and the universe)? Is this what he calls a 'singularity'? Which I could think like this: when there is no time nothing can happen, except during this singularity.  Of course I don't know of a law that says 'when there is no time nothing can happen..', I just came up with it, as something intuitive.
88584
Folks, I have a naive question regarding the subject of dark energy and an accelerating universe:  From what I understand/read, it seems that the further we look out into deep space, the faster the objects are moving away from us - in all directions.   Is this basically what is meant by "accelerating universe"?  Because it seems that this situation is exactly what we should expect to see (according to Big Bang theory). The further out we peer into space, the further back in time we are "seeing", so wouldn't we expect to see higher rates of acceleration/expansion the further back in time we peer?
62043
My question has a silly formulation, but I want to know if there is some sensible physical question buried in it:   Suppose an exact copy of our Universe is made, but where spatial distances and sizes are twice as large relative to ours. Would this universe evolve and function just as ours?   Since mass is proportional to volume which is distance cubed, but strength of a rope is only distance squared, I think not everything would scale proportionally.   Does this mean that a universe with our physical laws that evolves like ours can only have one particular size? Or would the physical laws scale proportionally so that it evolves in the same way? If it has a particular size, what is it relative to? Another variation: suppose another exact copy of our Universe is made, but where everything happened twice as fast relative to ours. Would it evolve in the same way as ours?
52838
I recently read that:     What our current level of thermodynamics does tell us though is that if there is such a thing as a Big Crunch, the universe will look a lot different as it heads towards collapse than it did when expanding – the universe started in a state of low entropy and high order and will end in a state of high entropy and   low order.   But shouldn't the order be in a high state and hence entropy becomes a low state in the Big Crunch (assuming a closed universe)?  The reason I write this is because the gravitational forces in a closed universe are so strong they will cause gravitational collapse so the universe will shrink and go backwards, eventually through all the stages of the big bang and become a singularity once more.   It was my understanding that a singularity has a state of high order and low entropy. Have I got this the wrong way round?   If so; Can someone please explain why?
15224
The size of our observable universe must have grown over time in the early universe.  Conversely with the accelerated expansion, I have heard that eventually our observable universe will shrink down to our local group of gravitationally bound galaxies.  So then the observable universe must go through a maximum at some point in time.  When is that and what is that maximum size?
18792
Did spacetime start with the Big Bang?  I mean, was there any presence of this spacetime we are experiencing now before big bang? And could there be a presence/existence of any other space-time before the big bang?
71963
Regarding this thread:    Why is the idea that the total energy in the universe "zero" so popular (re: Laurence Krauss) and why is the flatness of the universe used to back this up when, according to that post, an open universe would not conserve energy so the total energy of the universe cannot be "zero", can it? What's the energy being defined as zero and why is that energy used to predict things about the universe when "the" energy (as the term is used in that post) is not zero?  Additionally, I found one very good explanation here for this,    This seems to indicate that in flat space (the sort of space used in these zero energy universe theories) the mass can NOT be positive thus contradicting the notion that mass is positive and gravity negative and the whole thing winds up being zero. Did I interpret that correctly?  The MO question is cross-posted to Physics.SE here,
67446
Does the Friedmann vacuum equation have a linear solution rather than an exponential one?  Using natural units one can write Friedmann's equation for the vacuum as $$ \begin{eqnarray} \left(\frac{\dot a}{a}\right)^2 &amp;=&amp; \frac{8\pi G}{3}\rho_{vac}\\\tag{1} &amp;=&amp; L^2 \left(\frac{\rho_0}{L^4}\right) \end{eqnarray} $$  where I define the Planck length $L=(8\pi G \hbar / 3 c^3)^{1/2}$, $\hbar = c = 1$, and $\rho_0$ is a dimenionless constant.  Now let us interpret the Planck length $L$ to be the size of the smallest volume of space that can be described by general relativity.  But the Weyl postulate, together with cosmological observations, also imply that space is expanding.  Therefore we must have  $$L = a(t) L_0\tag{2}$$  where $L_0$ is the Planck length measured at the reference time $t_0$ where $a(t_0)=1$.  Inserting Eq.(2) into Eq.(1) we find  $$\left(\frac{\dot a}{a}\right)^2 = L_0^2 \left(\frac{\rho_0}{a^2L_0^4}\right)\tag{3}$$  where the Friedmann equation (3) has been rescaled in terms of the Planck length $L_0$ measured at the reference time $t_0$.  Eq.(3) has a linear solution  $$a(t) = \frac{t}{t_0}.$$  The scaled mass density $\rho(t)$ of the vacuum is not constant but rather given by  $$\rho(t) = \frac{\rho_0}{a^2 L_0^4} = \frac{1}{t^2 L_0^2}.$$
68685
The parameters of $\rm\Lambda CDM$ model have been determined to an amazing high precision from the measurements made by the Planck mission. In particular, the Hubble "constant" (the value of Hubble parameter at the present day) has been determined to be $H_0 = (67.3 \pm 1.2 )\, \text{km} \, \text{s}^{-1} \text{Mpc}^{-1}$. They have also given values for the density parameters at the present day (the density divided by the critical density) coming from cosmological constant, $\Omega_{\Lambda,0} = 0.6817$, and from matter, $\Omega_{m,0} = 0.3183$. The age of the Universe $t_0$ can be calculated from other cosmological parameters in this way  $$t_0 = \frac{1}{H_0}\int_0^1 \frac{a\,\text{d}a} { \sqrt{\Omega_{\Lambda,0}a^{4} + \Omega_{k,0}a^{2} + \Omega_{m,0}a + \Omega_{r,0}} },$$   where $\Omega_k = 1 - \Omega$ is the space curvature parameter, $\Omega = \Omega_{m,0} + \Omega_{\Lambda,0} + \Omega_{r,0}$ is the total energy density parameter (the energy density divided by the critical density) and $\Omega_r$ is the energy density parameter coming from radiation.  I have read that the age of the Universe has been established from the Planck mission measurements as $t_0 = 13.82 \times 10^9$ years. My question is: how is this value been calculated? I mean, it has been calculated assuming a flat space geometry, that is, assuming that $\Omega_k = 1 - \Omega = 0$? If not, what values for $\Omega_{r,0}$ and $\Omega_{k,0}$ have been used to perform the calculation?
100197
A theory among scientists says that quantum fluctuations caused the big bang and created the universe. This seems plausible to me.   What I can't grasp yet is how a quantum fluctuation can even start without an existing universe. Aren't all physical laws created with the universe? I understand that there is no notion of "before" with respect to time, however the big bang is theorised to have occurred, but for that to occur there must have first existed something right?  I wonder also, if there was a more nothingness instead of vacuum before the universe existed and how a quantum fluctuation could have started really from ex nihilo instead of a vacuum.
44215
As a start I assumed  which is the reciprocal of  as the frequency upper limit, but not so sure. On the other hand, is there a lower limit? Can we consider the reciprocal of our universe as a theoretical limit which is not yet limited, or just assume 0? I am just curious to know if you have any ideas with thanks.
81054
Is this a postulate? I get the expansion of the universe, the addition of discrete bits of space time between me and a distant galaxy, until very distant parts of the universe are moving relative to me, faster than the speed of light. But the other sure seems like  (giant steaming load of convenience) I think I get reasons why, because they don't have any other way to explain the size of the universe and so on. Just seems it would be much easier to  say the universe is eternal and cyclical in nature, and call it a day!