Universe what do we know

The physics at that era are of course still conjectural. The generic idea of inflation has achieved success in predicting two features of the fluctuations: that they are Gaussian, and that their amplitude depends on scale in a distinctive way. As well as generating the density fluctuations that evolve into galaxies, quantum effects could generate a second kind of fluctuation: gravitational waves that generate transverse motions, without changing the density. Now for another basic question: How much space is there altogether?

How large is physical reality? We can only see a finite volume, a finite number of galaxies. That is essentially because there is a horizon, a shell around us delineating the distance light could have travelled since the Big Bang. But that shell has no more physical significance than the circle that delineates your horizon if you are in the middle of the ocean. There is no perceptible gradient across the visible universe, which suggests that, if finite and bounded, it stretches thousands of times farther.

But that is just a minimum. If it stretched far enough, then all combinatorial possibilities would be repeated. Far beyond the horizon, we could all have avatars. But that is not all. Plausible models for GeV physics lead to so-called eternal inflation. But it is physics, not metaphysics; we can hope to push the casual chain back farther still. So a challenge for twenty-first-century physics is to address two fundamental questions.

First, are there many big bangs rather than just one? Second, if there are many, are they all governed by the same physics or not? Many string theorists do not think so.

They think there could be a huge number of different vacuum states — arenas for different microphysics. Many patches could be still-born or sterile: the laws prevailing in them might not allow any kind of complexity. We therefore would not expect to find ourselves in a typical universe; rather, we would be a typical member of the subset where an observer could evolve. This is sometimes called anthropic selection. Such conjectures motivate us to explore what range of parameters would allow complexity to emerge.

Those who are allergic to multiverses can regard this just as an exercise in counterfactual history rather as historians speculate on what might have happened to America if the British had fought more competently in , and biologists conjecture how our biosphere might have evolved if the dinosaurs had not been wiped out.

Anthropic arguments are irrelevant if the constants are unique. Otherwise, they are the best explanation we will ever have. It is reminiscent of planetary science four hundred years ago, even before Newton. At that time, Kepler thought that the Earth was unique, its orbit related to the other planets by beautiful mathematical ratios. We now realize that even within our own galaxy there are billions of stars, each with planetary systems.

Maybe we are due for an analogous conceptual shift on a far grander scale. Our Big Bang may not be unique any more than planetary systems are.

Mention of a multiverse often triggers the response that unobservable domains are not part of science. I want to contest this by way of aversion therapy, the psychological process of increased exposure whereby you are, for example, at first presented with a spider a long way away, but end up at ease even with tarantulas crawling over you.

However, as Wendy Freedman explained, we realize now that these galaxies are accelerating away from us, which means that they will never in principle be observable.

But since they will never be observable, why is their reality more acceptable than that of galaxies in the aftermaths of other big bangs if there are other big bangs, which we, of course, do not know?

If there is a multiverse, it will take our Copernican demotion one stage further: our Big Bang may be one among billions. It may disappoint some physicists if some of the key numbers they are trying to explain turn out to be mere environmental contingencies.

It is irrelevant to 99 percent of scientists. I have lived my life among astronomers, and I can assure you that their awareness of vast expanses of space and time does not make them more serene in everyday life. But there is one special perspective that astronomers can offer: an awareness of a vast future.

The stupendous time spans of the evolutionary past are now part of common culture. But most people still somehow think that humans are the culmination of the evolutionary tree. That hardly seems credible to astronomers. Our Sun formed 4. It will then flare up, engulfing the inner planets. The expanding universe will continue — perhaps forever — destined to become ever colder, ever emptier. Posthuman evolution — here on Earth and far beyond — could be as prolonged as the Darwinian evolution that has led to us, and could be even more wonderful.

And, of course, the evolution is even faster now: machines may take over. However, even in this concertinaed timeline — extending billions of years into the future, as well as into the past — this century may be a defining moment, for good or for ill.

It is the first century when complex entities — technologically empowered humans — have mapped the cosmos and have begun to understand how they emerged. This pale blue dot in the cosmos is a special place. It may be a unique place. And we are its stewards at a crucial era. That is a message for us all, whether we are interested in astronomy or not. Summer Bulletin.

Back to table of contents. Introduction Richard A. Meserve Wendy Freedman Martin Rees. Meserve is President of the Carnegie Institution for Science. Wendy Freedman. Wendy Freedman is Crawford H. Figure 1. Left: Astronomer Edwin Hubble examining plate, c. Figure 2.

Hubble Diagram We think Hubble did not actually believe that the universe was expanding, despite the evidence his empirical results provided. Martin Rees. Imagine sailing on a boat on the ocean and seeing a horizon in the distance, past which you know there is more Earth, but you just can't see it.

We've measured the universe to be flat as opposed to curved like Earth or saddle-shaped , but our horizon exists because of the finite speed of light. Beyond that visible horizon, we think the universe just keeps going in the same way - forever.

We have no reason to believe there is an edge. But we also have no way of measuring this infinity because we physically cannot see it. Only 5 per cent of the universe is made of ordinary material like planets, stars, cars, and coffee. This "normal matter" is made mostly of protons, neutrons, and electrons. Another 24 per cent is an exotic material that interacts through gravity, but produces no light, making it invisible to us. We call this "dark matter".

While dark matter only interacts with normal matter very weakly, particle physicists have plausible candidates for what dark matter is. Hopefully particle accelerators like the Large Hadron Collider will provide more insight for scientists very soon. That brings us to the final 71 per cent of the stuff in the universe, which is a truly bizarre type of matter.

Perhaps it's not matter at all, but a property of the universe itself. We call this mysterious stuff "dark energy". What we do know is that dark energy has a gravitationally repulsive effect that is causing the expansion of the universe to speed up. But we don't understand how this acceleration is happening. But the Big Bang should not be imagined as a normal explosion in space.

Rather, the Big Bang is an explosion of space itself, so that every point in space expands equally away from every other point in space. There is no centre to the expansion. From our galaxy we measure that all galaxies are moving away from us, and the farther the galaxy, the faster away it is moving.

The interesting thing is that if you zoomed off to any other galaxy in the universe, you would measure the exact same effect - all other galaxies would be moving away from you.

In this way, you could argue that you are the centre of the universe. But then, so is everyone else. When we look at distant galaxies, we are actually looking at a snapshot of the past. Our ten trillion human cells are augmented, exploited, nurtured, by a hundred trillion microbial cells - a couple of pounds of bacteria and archaea that we all carry around and can't live without.

They're in our guts, our lungs, up our noses and in every other dank corner. We're just cruise ships for the ultimate microbial Club-Med, and we simply don't know what that all means.

We don't know how the Earth works: Let's lurch back to a grander scale. No human, or robot, has ever physically traveled deeper than a few miles into the Earth's crust, everything else is extrapolation and interpolation from 'remote sensing' and clever physical analyses. It took us a ridiculously long time to figure out that the outer planetary skin is moving and sliding around; plate tectonics was not generally accepted until the midth century!

We're still not sure exactly how the inner dynamo works, how rolls of convecting, conducting material in the outer core generate our planetary magnetic field. There's also so much mess after 4. Speaking of other worlds, we're not even sure we understand where the Moon came from, maybe it was a giant impact, maybe not. For an allegedly clever species on a small rocky planet this is a bit of an epic fail. We can't prove or solve many of our own mathematical conjectures and problems: Ouch.

Lest mathematics thinks it can escape this festival of ignorance, just remind yourself that there's a long list of unproven, unsolved problems and unproven conjectures. Here, take a look. All in all, best kept firmly brushed under the carpet. Another glass of sherry professor?

We don't know how to make an artificial intelligence: I'm putting this here because it's a perennial problem, and one that speaks to both our desire to understand ourselves if you can make an artificial being you may find the secret sauce behind your own intelligence, even if ultimately it's just an emergent phenomenon as well as to understand what might be 'out there' in the vastness of the cosmos, wrought by billions of years of alien evolution, and really quite depressed by it all.

Although we've come a long way with our machines, it's not clear that predictive text or automated suggestions for shopping and movie streaming are really assembling information in any way that resembles how our minds generate ideas. This is truly a frontier.

The conclusion? There's an awful lot we don't know far more than just the examples here. But the point is not to get despondent, because this ignorance is a beautiful thing. It's what ultimately drives science , and it's what makes the universe truly awe-inspiring. After the hundreds of thousands of years that Homo sapiens has loped around, the cosmos can still elude our fidgety, inquisitive minds, easily outracing our considerable imaginations.

How wonderful. The views expressed are those of the author s and are not necessarily those of Scientific American. Caleb A. Scharf is director of astrobiology at Columbia University. He is author and co-author of more than scientific research articles in astronomy and astrophysics. For many years he wrote the Life, Unbounded blog for Scientific American.

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