A wonderful article from Aeon Magazine reminding us that it truly is infinite.
“Space, as they say, is big. In The Hitchhiker’s Guide to the Galaxy (1979), Douglas Adams elaborates: ‘You may think it’s a long way down the road to the chemist, but that’s just peanuts to space.’ It’s hard to convey in everyday terms the enormity of the cosmos when most of us have trouble even visualising the size of the Earth, much less the galaxy, or the vast expanses of intergalactic space. We often talk in terms of light-years – the distance light can travel in a year – as though the speed of light is somehow more intuitive than a number written in the trillions of kilometres. We give benchmarks in the same terms (it takes light 1.3 seconds to travel between the Earth and the Moon) but, in our everyday experience, light is instantaneous. We might as well talk about the height of a building in terms of stacking up atoms.
“Maybe, if we’re feeling more adventurous, we use analogies based on personal experience. The distance to the Moon is 32 million school buses! If you could drive there in one of those school buses, going at 60 miles per hour, it would take you 166 days! I’m not sure that helps.
“I wish I could say that astronomers have a better intuitive grasp of all this. We don’t. Brains don’t really work that way. So we cheat with numbers. We use longer yardsticks to talk about bigger spaces: kilometres, light-years, parsecs, kiloparsecs, megaparsecs, gigaparsecs. We get comfortable with exponents (1,000 is 103; 1 trillion is 1012) and think in logarithmic intervals, where each successive step is a new power of 10. At some point, distance stops being a straightforward concept entirely. Here in the Solar System, space and time are both more or less well-behaved, but when you have to deal with the cosmos as a whole, you have to factor in the fact that it refuses to sit still for its fitting.
“Space is expanding. It has been since the Big Bang, and it’s not stopping any time soon. If you look at a galaxy far, far away, not only do you have to factor in that the image you’re looking at is old, you have to account for the fact that it’s no longer where it was when you saw it. Let’s say you see a supernova go off, in a galaxy a billion light-years away. Did the supernova just go off, or did it go off a billion years ago? You can say the latter, because the light has been travelling to us for a billion years, but since there was no way to observe it back then, what does saying that it went off in the past even mean? And that billion-light-year-distant galaxy – how far away is it, really? Maybe a billion years ago it was a billion light-years away, but the Universe has been expanding all that time, so now it must be much farther. Which distance do we use?
“Even time is distorted by the stretching of space. We can watch the brightening and dimming of that exploding star, as the shockwave tears through it, and say it took about 100 days to fade away. But if we compare it with a supernova nearby, on average, we’ll see that the distant one takes a few days longer. From our perspective, it’s exploding in slow motion.
“Even with the limitations of definition, we do our best to measure our space and quantify its farthest reaches. We have catalogued countless galaxies, some so distant that their light has taken almost the entire lifetime of the cosmos to reach us. We have searched our maps of the cosmos for some indication of an edge, or a centre, and found none. We have no reason to believe the cosmos doesn’t just go on forever, in every direction, without any significant change in content or structure. Our galaxy is a single grain of sand in a vast unbroken desert; zoom out far enough, and everything looks more or less the same.
“There is a limit, though. However powerful our telescopes, and however long we stare, we will never see anything farther away than the edge of the cosmic bubble we call our ‘observable universe’. This is an imaginary sphere, centred on us, and defined by the speed of light and the age of the cosmos. The radius of this bubble is the distance that a beam of light could cover if it travelled for the entire age of the Universe.
“If every time we look out into the cosmos, we’re looking into the past, it stands to reason that looking far enough away could mean looking at the time so far into the past, it’s the moment when the Universe first formed. That’s what defines our cosmic horizon. Put another way, anything beyond our cosmic horizon is so far away that even if a light beam left it at the very moment the Universe started, 13.8 billion years ago, the distance is so vast that the light beam hasn’t had time to reach us yet. There hasn’t been enough time.
“We have good reason to believe that in this apparently boundless universe, there are galaxies beyond the horizon, just as, when you stand on the ocean shore and see nothing but water, you have reason to believe there’s land out there eventually, beyond what you can see. If you jumped in a ship and sailed away, your horizon would move with you, and you would eventually see that land. Similarly, if you could take off in an interstellar rocket ship to another part of the cosmos, your horizon would still be centred on you, wherever you were. Unfortunately, limited as we are by the laws of physics and the constraints of our modes of travel, getting far enough from home to significantly change our field of view isn’t practical. But we can still make inferences about what might lie beyond it. And despite the cosmic horizon being as subjective a boundary as a horizon is on Earth, it has one very important difference.
“When we look out to the edge of the observable universe, what we see is something truly astounding. The most distant light is also the oldest; it’s the light from the Big Bang itself. The early universe, right after the first moments of creation, was hot and dense, everywhere, humming with vibrating plasma; right at the edge of our vision, we’re looking into the past so far that we literally see that glowing plasma. The inferno persisted for around 380,000 years before space expanded and cooled enough that light and particles could travel freely through it. When we look at the edge of the observable universe, we see the last smouldering embers of that hot dense phase. We see a cosmos that is still on fire.”
Retrieved July 31, 2020 from https://aeon.co/essays/our-cosmic-horizon-is-both-unreachable-and-closer-than-ever
Be well everyone!