High School Expansion of the Universe

[petercl14]

TL;DR

accelerating expansion of the universe

Einstein's equations in general relativity indicate that the universe is expanding. Einstein himself believed that the universe should be stable so he introduced a correction to his equations which made for a static universe. He later admitted that was his biggest blunder. Hubble proved by observation that the universe was indeed expanding.
Latest observations are that the universe is expanding at an accelerating pace. Faster than in the past. Scientists are now trying to introduce a new phenomenon called dark energy to account for this.
As the universe expands galaxies become further apart. There is less gravitational attraction between them. Thus the universe will expand at an accelerating rate. There is no need to introduce another non entity like dark matter and call it dark energy.

 

[Bandersnatch]

As the universe expands galaxies become further apart. There is less gravitational attraction between them. Thus the universe will expand at an accelerating rate.

See, the first two sentences are fine. But the third doesn't follow - not without dark energy. Without dark energy a universe with any amount of mass in it must always decelerate, much like a ball thrown out of a gravity well must always decelerate even as the gravitational potential monotonically decreases. I.e. there is always some attraction, invariably acting to slow the galaxies down - because gravity can't be repulsive.
To get acceleration you need something repulsive to take over once the mass density goes down sufficiently. That's what dark energy does.

 

[PeterDonis]

As the universe expands galaxies become further apart. There is less gravitational attraction between them. Thus the universe will expand at an accelerating rate.

No, this is not correct. As can easily be seen from the Friedmann Equations, which are just the Einstein Field Equations for the case of a homogeneous, isotropic, expanding universe, a universe that only contains matter and radiation, with no dark energy, will have decelerating expansion.

 

[Hyku]

I did a google search for:
The Implications of Hubble's Law: An Expanding Universe
e-education.psu.edu

Here is the opening paragraph:
"Like Kepler's Laws, Hubble's Law is an empirical law. Hubble discovered a relationship between two measurable properties of galaxies: their velocities and their distances. Given this relationship, though, it naturally leads to several questions. These questions are:

• What is the cause of this relationship?
• Why should more distant galaxies have larger velocities?"

As a layman, I found the article's information to be informative, thought provoking, and relevant.

(Mods - if my post has violated any rules, I assure you that I am posting in good faith, and I ask for your understanding.)

 

[PeroK]

I did a google search for:
The Implications of Hubble's Law: An Expanding Universe
e-education.psu.edu

Here is the opening paragraph:
"Like Kepler's Laws, Hubble's Law is an empirical law. Hubble discovered a relationship between two measurable properties of galaxies: their velocities and their distances.

One difference between Kepler and Hubble is that Kepler took 20 years to work through an enormous amount of data on planetary motion before finally proving that the orbits were elliptical relative to the Sun. Hubble, on the other hand, took a huge guess based on a small sample of inconclusive data. The data that really confirms Hubble's law only came out years afterwards. But, by then, his was the name on it. The two sides of experimental science and gaining immortality!

 

[Jaime Rudas]

TL;DR Summary: accelerating expansion of the universe

As the universe expands galaxies become further apart. There is less gravitational attraction between them. Thus the universe will expand at an accelerating rate. There is no need to introduce another non entity like dark matter and call it dark energy.

Your argument fails for the same reason that the following argument fails:

"As a rocket ascends, it moves away from Earth. There will be less gravitational attraction between Earth and the rocket. Thus, the rocket will move away from Earth at an accelerating rate and will not require additional fuel expenditure."

 

[Drakkith]

Latest observations are that the universe is expanding at an accelerating pace. Faster than in the past.

I believe the current effect is that the deceleration of the expansion is slowing, not that the rate of expansion is increasing. A bit like gradually applying less force on the brakes when stopping your car. You decelerate hard at first, then less and less over time. The current rate of expansion is less than in the past.

@PeterDonis is that correct?

 

[PeterDonis]

I believe the current effect is that the deceleration of the expansion is slowing, not that the rate of expansion is increasing. A bit like gradually applying less force on the brakes when stopping your car. You decelerate hard at first, then less and less over time. The current rate of expansion is less than in the past.

@PeterDonis is that correct?

If by "the rate of expansion" we mean $\dot{a}$, i.e., the rate of change of the scale factor with respect to the proper time of comoving observers, and by "acceleration" we mean $\ddot{a}$, the rate of change of $\dot{a}$ with respect to that same proper time, then the acceleration now is positive, $\ddot{a} > 0$, and has been so for a few billion years. This is what we would expect if dark energy dominates the dynamics, i.e., if dark energy density is larger than all of the others combined (IIRC it's about twice as large now as all the others combined).

 

[PeterDonis]

The misconception in the OP has been corrected. Thread closed.

 

[Drakkith]

AFAIK, if by "the rate of expansion" we mean $\dot{a}$, i.e., the rate of change of the scale factor with respect to the proper time of comoving observers, and by "acceleration" we mean $\ddot{a}$, the rate of change of $\dot{a}$ with respect to that same proper time, then the acceleration now is positive, $\ddot{a} > 0$, and has been so for a few billion years. This is what we would expect if dark energy dominates the dynamics, i.e., if dark energy density is larger than all of the others combined (IIRC it's about twice as large now as all the others combined).

Ah, well nevermind then. I thought I had read otherwise.