01 November 2016

Some commentary on cosmology I sent to a friend

I got into a colloquy with a friend arising from my post about the Great Attractor. Which led to me trying to splain it a little bit more in depth. For those relatively few
​ who might be interested​
interested, here's what I
​wrote to her

> * She asked if the "Great Attractor was what it sounded like"  and was it in the picture I included in my post.

To which I replied,

Yes, although it's not a visible feature. The Great Attractor is a direction in space towards which everything in a large region including our galaxy, and the Virgo cluster BEHIND us, is being drawn. It was formerly thought to be an unusually massive region of galaxy concentration in that location (indicative of many supermassive black holes), but now there are other ideas, since the effect is so large. These range from even more massive concentrations behind that apparent point in space, possibly obscured by what's in front, to an incursion or leak in gravitational energy from outside the Big Bang universe. That suggestion is allowed by some versions of String Theory, but most cosmologists don't buy it. Most think it's an anomalous, but statistically not really outlandish, concentration of ordinary matter. Here is a version of that view:
> "
The Great Attractor is one such structure, a diffuse concentration of matter some 400 million light-years in size located around 250 million light-years (ly) away in the direction of the southern Constellation Centaurus, about seven degrees off the plane of the Milky Way."  [SolStation]

It's like the earth. It's 12000 km in diameter, but because it's spherical, even from as close as its surface, the gravitational force is even and acts as if all the mass were concentrated at a point, the center. Similarly, an enormous mass concentration approximately symmetrical in shape acts like a point source of gravitational attraction from a distance.
> *To which she replied:
> so it's an actual planet?  or it's postulated?
> that's so massy that it attracts, gravitationally, all the mass in the galaxies around it?
> I feel as if all this you say is like 'talking around what you want to say', instead of saying it.
> Is this you? or is this all the scientific writers?
> So when you say attractor, you mean gravitationally?  Or just that everything seems to moving in that direction, but it's not known why?
> Define your terms please.

And I responded:

Sorry. I have been interested in this subject since I was six years old so I speak the language and sometimes forget that others don't.

No it's not a planet. Wrong scale. We are talking about something that's 250 million light years distant and associated with the largest structures known in the universe. There are literally trillions of planets in these structures. The Great Attractor is a concentration of matter on an enormous scale that attracts galaxies tens of millions of light years distant, including our own. Yes. Gravitationally. Gravitation is the only known force that acts on these scales, although it's now believed that space itself exhibits a repulsive force analogous, but opposite,  to gravity that acts to drive the expansion of space, now known to be accelerating. So, no. Convergence (big crunch) has now been ruled out. The present universe will expand forever, becoming more and more attenuated, until it essentially evaporates, tens our hundreds of trillions of years in the future.

Remember that as recently as 1920 most astronomers believed our Galaxy WAS the entire universe. We now know there are at least 300 billion galaxies in the observable universe, and undoubtedly far more than that in the (vastly larger) portion of the universe that is already beyond the horizon of ever being observable (given the accelerating expansion of all of space in the Big Bing Universe).

The reference to the gravitational force on the surface of the earth was only an analogy, to explain how something very extensive and diffuse can act as if it were a point source of gravitational energy. Thepp Great Attractor is not a "thing," per se, it's the nexus of a large region of concentrated matter.

I don't know any other way to express these concepts. There are no other terms for such large scale phenomena.

And yes I am writing this, but of course it reflects my reading in the non technical astronomical literature. Like I said, this is an interest of mine and always had been.

And then later added,

To your specific question: "so, from the big bang, things spread out, but now they're converging back toward a point?"

Your question reflects that you aren't realizing the scale of the observable universe. Hubble has revealed that in any direction you look, the "deep field" shows galaxies at distances of 10+ billion light years (which means we're looking at galaxies as they were when the universe, now 13.7 billion years old, was only less than four billion years old). The Laniakea Cluster, and (probably) the Great Attractor, are local phenomena on this scale, hundreds of millions of light years as opposed to quite a few billion light years distant.

The actual distance to the "Attractor" is not known, however. The assumption is that it is a concentration of matter in the densest part of our local Supercluster, but it's very difficult to say for certain that it isn't from even larger and more powerful sources of gravitational energy that are further, possibly much further, away, in the same direction.

But in any case the motion of everything around here, on a scale of tens of millions of light years to a few hundred million light years, towards the Great Attractor, is only an overlay on the continuous and accelerating expansion of space itself. As it turns out, we are living in an epoch when the universe is largely observable. We can almost see back to the time when the universe first became transparent, and we can detect the energy of the that time as the cosmic background radiation. (Only less than a million years after the Big Bang). Due to cosmic inflation, which is a whole other subject, most of the universe is and has been from near the beginning already too far away for light from those regions to ever reach here (and the same applies everywhere; observers anywhere can only see a small fraction of the universe). But that horizon will shrink as the accelerating expansion of space continues. Eventually even galaxies now visible at a few billion light years distant will be moving away from us faster than light travels (this is possible because space doesn't move, it expands). Their light will then never reach us. And this horizon will get closer and closer. In something like 10 billion years, which is a long, long time of course but less than the time the universe has already existed, and there will still be stars shining then, this horizon will essentially reach the position of a local observer. Anyone alive at that time will be able to see only their own galaxy and any local objects actually gravitationally bound to it; all other galaxies will be moving with the expansion of space faster than light from their stars could ever reach their eyes.

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