# How did inflation lead to the expansion of space?

Big bang Theory gives a scientific explanation of the evolution of the Universe, where the scientific picture of it starts with Universe(space) uniformly filled with matter and energy, already expanding without any description of how that expansion started. Alan Guth in this course states inflation as a Bang which Big bang theory doesn't give any description about. According the theory of cosmic inflation Universe undergone exponential expansion which was due to repulsive gravity between the patch of gravitationally repulsive unstable material which decayed like a radioactive substance ending the exponential expansion to get our local Universe to the state where the Big bang theory gives the solution from (where the space is already expanding). General relativity explains the Gravity as the geometry of Spacetime. If repulsive gravity is thought to be working the same way then how did gravity make the space continue expanding (as we mean by metric expansion) and how the space is expanding even today or If I am wrong how repulsive gravity work as the theory of inflation explains. I'm asking this as I previously mentioned Alan Guth called the inflation as Bang to the Big Bang or the event that drove the expansion of space.

• very nice Alan Guth quizzes with solutions ... but : Your search - repulsive gravity - did not match any documents. No pages were found containing "repulsive gravity". – user46925 Dec 17 '15 at 17:52
• Just go through these slides just from the first lecture. – user73541 Dec 17 '15 at 18:06
• yes, yes , it is explicite in this Guth document ! I'll first learn what he is talking about ( dark energy repulsion ? ) and why he calls it gravity. Thank you – user46925 Dec 17 '15 at 18:11
• Alan Guth wrote : The repulsive gravity material is unstable,so it decayed like a radioactive substance, ending inflation. At 10^-35 s... Does he mean that the laws of the repulsive gravity are somehow related to GR ? is the expansion necessarily connected to inflation ? – user46925 Dec 18 '15 at 4:57
• @igael you can go through the first video lecture from the course whose link I mentioned above in question and I have too mentioned in the question that Alan Guth called inflation as a Bang in big bang thus I asked the relation of inflation to expansion of space. – user73541 Dec 18 '15 at 6:14

Big bang Theory gives a scientific explanation of the evolution of the Universe, where the scientific picture of it starts with Universe(space) uniformly filled with matter and energy, already expanding without any description of how that expansion started.

Big Bang is a theoretical model within the theory of General Relativity. It was developed to explain the observation that all the clusters of galaxies are receding from each other and from us. This behavior is describable by an explosion in four dimensional space.

Extrapolation of the expansion of the universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past. This singularity signals the breakdown of general relativity and thus, all the laws of physics.

Forty years ago when I took a semester of cosmology that was the dominant model. Since then new observations showed that the model was inconsistent with the Cosmic Microwave Background observations, which showed a surprising uniformity of the matter distribution at the time of the decoupling of the photons, at 380.000 years from the beginning of the Universe. The uniformity could not be fitted thermodynamically because at that age not all parts of the universe could interact with others, due to the light cones of special relativity. Alan Guth in this course states inflation as a Bang which Big bang theory doesn't give any description about. According the theory of cosmic inflation Universe undergone exponential expansion which was due to repulsive gravity between the patch of gravitationally repulsive unstable material which decayed like a radioactive substance ending the exponential expansion to get our local Universe to the state where the Big bang theory gives the solution from (where the space is already expanding).

The model now , at times smaller than 10^-32 seconds has incorporated an effective quantization of gravity (quantization of gravity is an ongoing theoretical research project). This introduces randomization of the available energy density by a particle called "inflaton" , and small inhomogeneities also appear due to the intrinsic probabilistic nature of quantum mechanics; this develops into the CMB spectrum and the matter distribution we observe at present.

In the figure, the original Big Bang solution would start after the inflation period. Measurements since 1990 show that the universe is continuing in accelerated expansion, (seen in the figure as an opening horn shape) so the Big Bang name is just a historical terminology in the development of models for the universe, which is retained as a historical relic.

Expansion, the inflation periods, can be modeled with "negative gravity" mathematically, as gravity is attractive and leads to contractions.

General relativity explains the Gravity as the geometry of Spacetime. If repulsive gravity can be thought the same way the how did gravity make the space continue expanding (as we mean by metric expansion) and how the space is expanding even today or If I am wrong how repulsive gravity work as the theory of inflation explains.

There are two models: the underlying general relativity one of gravity as distortion of space time, and the other is to view the emergent effects as Newtonian gravity, since locally one can always approximate with Newtonian mechanics the geometry of general relativity. Gravity effects induce accelerations towards contraction. The inflation period is accelerated expansion. Thus since normal gravity is attractive, negative gravity is repulsive: the effect of accelerating expansion translated to a Newtonian mathematical framework can be modeled as negative gravity .

The observed continuing acceleration of the expansion of the universe, as seen in the figure, has introduced dark energy in the mathematical models, which has in newtonian mathematics a negative gravity effect.

In physical cosmology and astronomy, dark energy is an unknown form of energy which is hypothesized to permeate all of space, tending to accelerate the expansion of the universe. Dark energy is the most accepted hypothesis to explain the observations since the 1990s indicating that the universe is expanding at an accelerating rate

I'm asking this as I previously mentioned Alan Guth called the inflation as Bang to the Big Bang or the event that drove the expansion of space.

Yes, as you can see in the attached figure of the time development.

• You mentioned in your answer that behaviour of the Universe that we do see or the expansion can be described as an explosion in four dimensions. What would be the difference between a bomb explosion and an explosion like mentioned happening in four dimension. And how this expansion we do see can be aftermath of the inflation ? If I imagine gravity as a repulsive force (treating it as Newtonian gravity) then how that event(inflation) is making the 'space' between two points expand today. I'm unable to understand how inflation can lead to metric expansion of space. – user73541 Dec 18 '15 at 2:56
• If you take two masses isolated, their positive attraction means that the space between them diminishes with an acceleration given by Newtons laws. We call that positive gravity. The label of "negative gravity" was given to the observation of the need for an accelerated repulsion that pushes masses away. The difference between a Bang in space and time disconnected, or connected with General Relativity is that in the first case the mathematical model is r(t), theta(t) phi(t). There exists a center, r=0 for t=0. In General Relativity (x,y,z,t) is one locked vector and there exists a solution – anna v Dec 18 '15 at 5:16
• where all (x,y,z) are receding from each other as a function of t. this can be mathematically extrapolated to the (0,0,0,0) point which was the original big bang function. The center of the "explosion exists only at that one point. The balloon analogue might help for visualization. Blowing the balloon , all the points on the surface recede from each other. Thus an explosion in three space can be approximated by consecutive surfaces starting from a point. But on the two dimensional surface of the balloon exists the analogue of the three dimensional recession we have observed. – anna v Dec 18 '15 at 5:22
• Have a look here oneminuteastronomer.com/6949/… – anna v Dec 18 '15 at 5:26