In a recent article in The Guardian (Universe is expanding up to 9% faster than we thought, say scientists) the topic of the universe expansion is accelerating is brought up again. Astrophysicists are now saying that far galaxies are moving away form us from 5-9% faster than expected. As I understand this they are measuring the speed of far galaxies and finding them moving faster and surmising that those galaxies would have had to have sped up to get to their current. The problem I have is what does “was expected mean.”
The scientific story is that the universe, not just matter and energy, but also space have been expanding at quite a clip since the beginning of the universe 13.8 billion years ago. Opposing this expansion is drag in the form of gravitational attraction between the matter (once it formed, it took a while). So that the longer a galaxy has been around, the more drag it will experience and the slower it would go. It wasn’t that long ago that scientists consider a scenario in which the gravitational attraction would slowly overcome the original impetus supplied by the “Big Bang” and the expansion would stop, reverse it self and all of the stuff of the universe would pull itself back together in what was called the “Big Crunch.”
Opposing the possibility of the Big Crunch scenario was the universe would expand forever scenario and several others. What was needed was data to distinguish between the competing scenarios.
As it turned out, the Big Crunch does not seem to work, currently most think the universe keeps expanding forever, but the question is how.
The idea of gravitational drag says something about what is going on. Since galaxies on the “edge” of the universe (those first created and moving longest) are now farther apart from other galaxies, the drag due to the gravitational pull of those other galaxies is less, so they should be slowing less that galaxies closer to the center of the universe. The galaxies on the “edge” are not only getting farther from the center but also farther from those galaxies to either side. This we were taught in grade school. The usual demonstration was to put to ink dots on a balloon and show that they get farther apart from one another as the balloon is inflated. The area of the sphere (centered on the center of the universe) gets larger as the square of the radius so the sideways expansion is as significant if not more than the expansion from the center of the universe.
So, since the galaxies farther away have been slowed at a lower rate (a lower deceleration) than closer galaxies, should they not be a tiny bit faster … ?
Or … did those galaxies have to go through stages like the ones closer in, so as to have the same effect on all?
This is very confusing. In order for an actual acceleration be going on, there must be some unseen “pushing” force, never encountered before (the so-called dark energy) or an unseen pull force never seen before (the so-called dark matter), or possibly the rate of space-time expanding is itself undergoing a rate change. I have not read anyone who has a handle on the expansion of space-time during the Big Bang so I suspect no one is an expert on a possible contraction or acceleration of it now.
Anyone? Anybody out there understand this?
(I suspect Sean Carroll might but currently I am ticked off with Dr. Carroll, which I will explain in my next post.)