Observable Universe and speed of light

ps200306

Justin1982 wrote: »

Hopefully this explanation helps guys.

There was a big bang most likely, there may have been a rapid inflationary period (pinch of salt), the universe existed for something like 300,000 years (again more salt for easier digestion). But all that doesn't matter.

Why?

Well because during that 300,000 years after big bang light could not propagate they way we think of it today. Universe was too dense. So once it expanded enough then light could propagate in its normal sense. Again this light started propagating 300,000 years after the big bang. So one of the photons of light that started propagating freely traveled through the universe unimpeded for billions and billions and billions of years. Just travelling and travelling and travelling for about 13.798±0.037 billion years until right just now it hit you on the head.

Personally that's the way I like to think of the age of the universe and the cosmic horizon. All the stuff before the cosmic horizon cannot be seen visually because light was basically not around at that stage. And what went on before it is kind of irrelevant when talking about the age of the observable universe.

The 300,000 years is just a rounding error though when we're talking about a 13.7 Gyr age for the universe. Lopping 300 Ky off that puts the recombination era (when photons first started travelling) at 13.6997 Gyr ago (except the 300 Ky is actually a hundred times smaller than the uncertainty in the age, so that level of precision is unwarranted). Things like cosmic inflation do have an observational implication, albeit indirect. Regions at the cosmic horizon that are separated by (IIRC) 7.5 degrees on our sky aren't causally connected, so we need something like inflation to explain the homogeneity and isotropy of the universe.

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so how does gravity leave a black hole , if it cannot travel faster than light ?

it must be instant , or much faster than light .

ps200306

DaDumTish wrote: »

so how does gravity leave a black hole , if it cannot travel faster than light ?

it must be instant , or much faster than light .

A couple of ways to look at this ... one answer is "same as light leaves a black hole at the speed of light". Ok, light can't escape a black hole but it can escape a point arbitrarily close to the event horizon. To be sure, it will be increasingly gravitationally red shifted with proximity to the event horizon, but it nevertheless propagates at the speed of light from the point of view of all observers outside the black hole.

Another point is that gravity doesn't "leave the black hole" in the sense of something flowing out of it. Gravity is the curvature of space around the black hole. The gravity well formed long before the black hole, under the influence of whatever object was the black hole's progenitor. As the black hole formed, the gravity well got increasingly steep.

Imagine you're standing on the surface of the progenitor star, with a pal beside you. The surface of the star falls away as the collapse occurs and your pal goes with it. (We'll ignore that it's the core, not the surface, of a star that collapses). You have the presence of mind to have turned on your super-powerful hover boots, so manage to resist the star's surface gravity. (The force on you doesn't change as the black hole forms, as both the mass below you and your distance from the centre stay the same).

You are in radio contact with your less fortunate pal as he disappears down the drain. He somehow manages to avoid spaghettification and measures the increasing force with his gravitometer all the way down, radioing the results back to you. It's digital communication in the form of little bursts of wavepackets. His radio signal is increasingly redshifted as he falls, so it takes longer and longer for a sequence of packets to arrive. Initially your pal is transmitting a reading every microsecond, and you see that the gravitational force is rapidly soaring. Later, there are larger and larger gaps between readings. They stretch to seconds, then minutes, hours, and days. Eventually, although the signals keep coming, they take years to read, then thousands and millions of years.

Now, this is completely at odds with your pal's experience. From the start of collapse to the formation of the event horizon takes seconds at most. His fall rate is limited only by the speed of light as measured by himself. We don't know what happens as the event horizon forms beside him, but we know it's all over in a flash. You and your pal's regions of the universe see things in each other's vicinities happening at completely different rates!

This is true not just for what we generally think of as "informational processes" (such as your pal's gravity readings) but for all of physics. The universe at your location doesn't know that the black hole has formed. The gravitational well -- i.e. the curvature of space in your vicinity -- persists because it doesn't "know" it's not supposed to. It doesn't need anything to leave the black hole, in fact it depends on that not happening! There are no naked singularities.