Alan Guth Quotes (1947-)

“Inflation is a prequel to the conventional Big Bang theory. ...It does provide a theory of the propulsion that drove the universe into this humungous episode of expansion which we call the Big Bang.”

“The conventional Big Bang theory says nothing about where all the matter came from. The theory really assumes that for every particle that we see in the universe today, there was, at the very beginning, at least some precursor particle, if not the same particle, with no explanation of where all those particles came from.”

“We should not act like we know that the universe began with the Big Bang... we'll see that there are strong suggestions that the Big Bang was perhaps not really the beginning of existence, but really just the beginning of our local universe, often called a pocket universe.”

“A positive pressure produces an attractive gravitational field... Positive pressures are just sort of normal pressures and attractive gravity is normal gravity, so normal pressures produce normal gravity, but it is possible to have negative pressures, and negative pressures produce repulsive gravity, and that's the secret of what makes inflation possible.”

“The conventional Big Bang theory does not say anything about what caused the expansion. It really is only a theory about the aftermath of a bang. In the scientific version of the Big Bang, the universe starts with everything already expanding, with no explanation of how that expansion started... So the Scientific version of the Big Bang theory is not really a theory of a bang, it's really a theory of the aftermath of a bang.”

“What it takes to produce a gravitational repulsion is a negative pressure. According to general relativity, it turns out... both pressures and energy densities can produce gravitational fields, unlike Newtonian physics, where it's only mass densities that produce gravitational fields.”

“Inflation takes advantage of this possibility... to let gravity be the repulsive force that drove the universe into the period of expansion that we call the Big Bang. In fact, when one combines general relativity with conventional ideas, now, in particle physics there really is a pretty clear indication, I should say, not quite a prediction... that at very high energy densities one expects to find states of matter which literally turn gravity on its head and cause gravity to become repulsive.”

“It turns out that the energy of a gravitational field—any gravitational field—is negative. During inflation, as the universe gets bigger and bigger and more and more matter is created, the total energy of matter goes upward by an enormous amount. Meanwhile, however, the energy of gravity becomes more and more negative. The negative gravitational energy cancels the energy in matter, so the total energy of the system remains whatever it was when inflation started—presumably something very small. ...This capability for producing matter in the universe is one crucial difference between the inflationary model and the previous model.”

“The gravitational repulsion created by this small patch of repulsive gravity material would be, then, the driving force of the Big Bang and it would cause the region to undergo exponential expansion... there is a certain doubling time, and if you wait the same amount of time it doubles again, and if you wait the same amount of time it doubles again... and it's because these doublings build up so dramatically, it doesn't take very much time to build the whole universe. In about 100 doublings this tiny patch of 10^-28 cm can become large enough, not to be the universe, but to be a small marble-sized region which will then ultimately become the observed universe, as it continues to coast outward after inflation ends.”

“A very plausible choice for when inflation might have happened would be when the energy scales of the universe were at the scale of grand unified theories... which unify the weak, strong and electromagnetic interactions into a single unified interaction. ..we're talking about energies which are about 10¹⁶ times the equivalent energy of a proton mass. ...the initial patch would only have to be the ridiculously small size of about 10^-28 cm across to be able to lead ultimately to the creation of everything that we see on the vast scale of which we see it.”

“The miracle of physics that I'm talking about here is something that was actually known since the time of Einstein's general relativity; that gravity is not always attractive. Gravity can act repulsively. Einstein introduced this in 1916... in the form of the cosmological constant, and the original motivation of modifying the equations of general relativity to allow this was because Einstein thought that the universe was static, and he realized that ordinary gravity would cause the universe to collapse if it was static. ...The fact that general relativity can support this gravitational repulsion, still being consistent with all the principles that general relativity incorporates, is the important thing which Einstein himself did discover..”

“What the Big Bang theory tells us, is that at least our region of the universe 13.82 billion years ago, was an extremely hot, dense uniform soup of particles which in the conventional standard Big Bang model filled literally all of space—and now we certainly believe that it filled essentially all of the space that we have access to—uniformly. ...This is contrary to a popular cartoon image of the Big Bang, which is just plain wrong. The cartoon image of the Big Bang is the image of a small egg of highly dense matter that then exploded and spewed out into empty space. That is not the scientific picture of the Big Bang. ...If there was a small egg that exploded into empty space, you would certainly expect that today you would see something different if you were looking towards where the egg was, versus looking the opposite direction, but we don't see any effect like that. When we look around the sky the universe looks completely uniform, on average, in all directions, to a very high degree of accuracy... So we don't see a sign of an egg having happened anywhere. Rather, the Big Bang seems to have happened everywhere, uniformly.”