It’s possible that the universe isn’t uniform past what we can see, and conditions are wildly different from place to place, says Caltech astrophysicist Sean Carroll. “That possibility is the cosmological multiverse. We don’t know if there is a multiverse in this sense, but since we can’t actually see one way or another, it’s wise to keep an open mind.”
“Astronomers estimate that the observable universe — a bubble 14 billion light-years in radius, which represents how far we have been able to see since its beginning — contains at least two trillion galaxies and a trillion trillion stars,” writes Dennis Overbye in New York Times Science. “Most of these stars and galaxies are too far and too faint to be seen with any telescope known to humans.”
“Because we can only see so far,” says Carroll, “we’re not sure what things are like beyond our observable universe. The universe we do see is fairly uniform on large scales, and maybe that continues literally forever.”
Fred Adams, theoretical cosmologist at the University of Michigan, wrote in an email to The Daily Galaxy, “We live within what is called the observable universe, which is the volume of the universe where `one thing can affect another’, and within this volume we see the universe to be uniform. Significantly, we can actually do experiments within this volume and those experiments (so far) tell us that the whole volume is extremely uniform.”
From our tiny blue water planet, the universe appears inconceivably vast. In the grand cosmic scheme of things, all the light in the observable universe provides about as much illumination as a 60-watt bulb seen from 2.5 miles away, says Marco Ajello, an astrophysicist at Clemson University, who led a team that has measured all of the starlight ever produced throughout the history of the observable universe.
A Bubble with a Diameter of 27.4 billion light years
The observable Universe is a bubble centered on the Earth, with a diameter of 27.4 billion light years – a bubble growing in size at a rate of two light years (one on each side) every year. The universe extends beyond our cosmic horizon, just as the sea extends beyond the sailor’s horizon, and may well (unlike the ocean) be infinite. The great mystery that will perhaps never be answered is what lies beyond the cosmic horizon.
On the basis of observations made with instruments such as the Hubble Space Telescope, it is estimated that there are hundreds of billions, and perhaps trillions, of galaxies in the observable Universe. But this observable domain, writes the great British astrophysicist Martin Rees, “may not be all of physical reality; some cosmologists speculate that ‘our’ big bang wasn’t the only one—that physical reality is grand enough to encompass an entire ‘multiverse’.”
Even conservative astronomers are confident that the volume of space-time within range of our telescopes—what astronomers have traditionally called ‘the universe’—is only a tiny fraction of the aftermath of the Big Bang. We’d expect far more galaxies located beyond the horizon, continues Rees, “unobservable, each of which (along with any intelligences) will evolve rather like our own.”
More of the Same?
We may, by the end of this century, concludes Rees, be able to ask whether or not we live in a multiverse, and how much variety its constituent ‘universes’ display. The answer to this question will determine how we should interpret the ‘biofriendly’ universe in which we live, sharing it with any aliens with whom we might one day make contact.
The edge of the observable universe is the place beyond which light hasn’t had time to reach us since the beginning of the universe, says Jo Dunkley, Professor, Physics and Astrophysical Sciences, Princeton University, whose research is in cosmology and studying the origins and evolution of the Universe. “That’s only the edge of what we can see, and beyond that is probably more of the same stuff that we can see around us: super-clusters of galaxies, each enormous galaxy containing billions of stars and planets.”
Or Wildly Different from Place to Place
Or maybe, as Sean Carroll says, it’s possible that the universe isn’t uniform past what we can see, and conditions are wildly different from place to place. A place with more of the same, or a terra incognita with dragons and sea monsters.
Fred Adams concluded in his email to The Daily Galaxy, “The observable universe, as defined above, is part of a larger volume. The region just outside the observable universe is expected to be uniform as well. Here, we *expect* the uniformity, based on theoretical considerations, but we cannot do experiments to show that this is the case. On still larger volumes, much much larger than the observable volume, we expect the regions to be less uniform. On this larger scale, it is indeed possible for the conditions to `vary wildly from place to place’ as you say. Significantly, we are saying that it is *possible* for the conditions to vary wildly, and we are *not* saying that they are known to vary.”
The image at the top of the page took researchers at the Instituto de Astrofísica de Canarias almost three years to produce this deepest image of the Universe ever taken from space, by recovering a large quantity of ‘lost’ light around the largest galaxies in the iconic Hubble Ultra-Deep Field.
Your free daily email of stories of space and science –a random journey from Planet Earth through the Cosmos– that has the capacity to provide clues to our existence and add a much needed cosmic perspective in our Anthropocene epoch.