Accelerating Expansion of the Universe

[Devil Particle]

Hey,

Congradulations to Saul Perlmutter, Adam Riess, and Brian Schmidt for their Nobel Prize.

Relating to this discovery which I know has been around for a while, does the accelerating expansion rate of the universe mean that as we "turn the clock backward" the rate of expansion decelerates? Does this imply there might be a point in the past where the rate of expansion slows to zero? Does this pose any threat to inflation theory?

[Chris Peterson]

Relating to this discovery which I know has been around for a while, does the accelerating expansion rate of the universe mean that as we "turn the clock backward" the rate of expansion decelerates? Does this imply there might be a point in the past where the rate of expansion slows to zero? Does this pose any threat to inflation theory?

The Universe has been expanding since the beginning. After all the weirdness associated with the early inflationary period, the Universe expanded, but that rate of expansion was decreasing (because the self-gravity of mass-energy was dominant- primarily that of dark matter). A few billion years ago, the mass-energy density dropped low enough that dark energy became dominant, and the rate of expansion began to increase.

I'm not sure exactly what you are referring to as "inflation theory", but this cosmological sequence of inflation, matter dominated expansion, then dark energy dominated expansion is at the heart of our current understanding of the Universe.

[Devil Particle]

Chris Peterson wrote:
A few billion years ago, the mass-energy density dropped low enough that dark energy became dominant, and the rate of expansion began to increase."

Have astronomers been able to observe the point in the universe's history when this occurred?

[Chris Peterson]

Chris Peterson wrote:
A few billion years ago, the mass-energy density dropped low enough that dark energy became dominant, and the rate of expansion began to increase."

Have astronomers been able to observe the point in the universe's history when this occurred?

At about redshift z=0.5 the acceleration is zero. At larger redshifts it is negative, and at smaller ones positive. In other words, the rate of expansion of the Universe was decreasing until about 5 billion years ago, was then constant, and has been increasing ever since.

[Devil Particle]

Chris Peterson wrote:

At about redshift z=0.5 the acceleration is zero. At larger redshifts it is negative, and at smaller ones positive. In other words, the rate of expansion of the Universe was decreasing until about 5 billion years ago, was then constant, and has been increasing ever since.

Chris,

Is this because dark energy is proportional to the amount of empty space in the universe and does this mean that the amount of dark energy is increasing?

[Chris Peterson]

Chris,

Is this because dark energy is proportional to the amount of empty space in the universe and does this mean that the amount of dark energy is increasing?

As space expands the force of gravity between distant objects gets smaller, but the force from dark energy does not (or, depending on the model, gets smaller at a lower rate). Related to this is that the density of matter (which is mostly dark matter) drops faster as space expands than the density of dark energy. Dark energy could not dominate until the density of matter was low enough that gravity ceased to be the dominant force.

[Ann]

Chris understands this better than I do, so I should probably not butt in. Like Chris said, gravity has not grown weaker, but the force of gravity decreases with distance. Distances in the universe have always been increasing due to the fact that the universe was expanding from the moment it was born in the Big Bang. The initial force of the Big Bang was very strong and "opened up a space" which was fast increasing in size. Due to the mass inside this space, the expansion slowed, but it never stopped. At a certain point in time distances had grown so large that gravity was no longer able to slow down the expansion. The "universal braking" had stopped.

Like Chris said, dark energy has been there all the time. But when the universe was small and all things were rather close together, gravity exerted a much greater force on the universe than dark energy. But as the universe kept expanding and things "flew farther and farther apart", gravity was less and less able to counteract dark energy, and therefore the relative strength of dark energy grew stronger. And dark energy makes the universe expand faster.

The way I understand it, the force of gravity has always been the same, and the strength of dark energy has also always been the same. But because the universe was born with an "impetus" that makes things fly apart, the influence of gravity has become weaker while the influence of dark energy has become stronger.

Ann