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One can not ask whether the model represents reality, only whether it works. A model is a good model if first it interprets a wide range of observations, in terms of a simple and elegant model. And second, if the model makes definite predictions that can be tested and possibly falsified by observation.

In terms of the positivist approach, one can compare two models of the universe. One in which the universe was created last year and one in which the universe existed much longer. The Model in which the universe existed for longer than a year can explain things like identical twins that have a common cause more than a year ago. On the other hand, the model in which the universe was created last year cannot explain such events.

So the first model is better. One can not ask whether the universe really existed before a year ago or just appeared to. In the positivist approach, they are the same. In an unchanging universe, there would be no natural starting point. The situation changed radically however, when Edwin Hubble began to make observations with the hundred inch telescope on Mount Wilson, in the s. Hubble found that stars are not uniformly distributed throughout space, but are gathered together in vast collections called galaxies.

By measuring the light from galaxies, Hubble could determine their velocities. He was expecting that as many galaxies would be moving towards us as were moving away. This is what one would have in a universe that was unchanging with time. But to his surprise, Hubble found that nearly all the galaxies were moving away from us. Moreover, the further galaxies were from us, the faster they were moving away. The universe was not unchanging with time as everyone had thought previously. The distance between distant galaxies was increasing with time.

The expansion of the universe was one of the most important intellectual discoveries of the 20th century, or of any century. It transformed the debate about whether the universe had a beginning.

Bubble universe

If galaxies are moving apart now, they must have been closer together in the past. If their speed had been constant, they would all have been on top of one another about 15 billion years ago. Was this the beginning of the universe?

Many scientists were still unhappy with the universe having a beginning because it seemed to imply that physics broke down. One would have to invoke an outside agency, which for convenience, one can call God, to determine how the universe began. They therefore advanced theories in which the universe was expanding at the present time, but didn't have a beginning. In the Steady State theory, as galaxies moved apart, the idea was that new galaxies would form from matter that was supposed to be continually being created throughout space.

The Origin of the Universe

The universe would have existed for ever and would have looked the same at all times. This last property had the great virtue, from a positivist point of view, of being a definite prediction that could be tested by observation. The Cambridge radio astronomy group, under Martin Ryle, did a survey of weak radio sources in the early s. These were distributed fairly uniformly across the sky, indicating that most of the sources lay outside our galaxy. The weaker sources would be further away, on average. The Steady State theory predicted the shape of the graph of the number of sources against source strength.

But the observations showed more faint sources than predicted, indicating that the density sources were higher in the past. This was contrary to the basic assumption of the Steady State theory, that everything was constant in time. For this, and other reasons, the Steady State theory was abandoned. Another attempt to avoid the universe having a beginning was the suggestion that there was a previous contracting phase, but because of rotation and local irregularities, the matter would not all fall to the same point. Instead, different parts of the matter would miss each other, and the universe would expand again with the density remaining finite.

Two Russians, Lifshitz and Khalatnikov, actually claimed to have proved, that a general contraction without exact symmetry would always lead to a bounce with the density remaining finite. This result was very convenient for Marxist Leninist dialectical materialism, because it avoided awkward questions about the creation of the universe. It therefore became an article of faith for Soviet scientists. When Lifshitz and Khalatnikov published their claim, I was a 21 year old research student looking for something to complete my PhD thesis.

I didn't believe their so-called proof, and set out with Roger Penrose to develop new mathematical techniques to study the question. We showed that the universe couldn't bounce. If Einstein's General Theory of Relativity is correct, there will be a singularity, a point of infinite density and spacetime curvature, where time has a beginning. Observational evidence to confirm the idea that the universe had a very dense beginning came in October , a few months after my first singularity result, with the discovery of a faint background of microwaves throughout space.

These microwaves are the same as those in your microwave oven, but very much less powerful. They would heat your pizza only to minus point 3 degrees centigrade, not much good for defrosting the pizza, let alone cooking it. You can actually observe these microwaves yourself.

Is the Universe a Bubble? – Physicists Work on the Multiverse Hypothesis

Set your television to an empty channel. A few percent of the snow you see on the screen will be caused by this background of microwaves. The only reasonable interpretation of the background is that it is radiation left over from an early very hot and dense state. As the universe expanded, the radiation would have cooled until it is just the faint remnant we observe today. Although the singularity theorems of Penrose and myself, predicted that the universe had a beginning, they didn't say how it had begun.

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The equations of General Relativity would break down at the singularity. Thus Einstein's theory cannot predict how the universe will begin, but only how it will evolve once it has begun. There are two attitudes one can take to the results of Penrose and myself. One is to that God chose how the universe began for reasons we could not understand. This was the view of Pope John Paul. At a conference on cosmology in the Vatican, the Pope told the delegates that it was OK to study the universe after it began, but they should not inquire into the beginning itself, because that was the moment of creation, and the work of God.

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I was glad he didn't realize I had presented a paper at the conference suggesting how the universe began. I didn't fancy the thought of being handed over to the Inquisition, like Galileo. The other interpretation of our results, which is favored by most scientists, is that it indicates that the General Theory of Relativity breaks down in the very strong gravitational fields in the early universe. It has to be replaced by a more complete theory.

Eternal inflation

One would expect this anyway, because General Relativity does not take account of the small scale structure of matter, which is governed by quantum theory. It consisted of two twelve-foot metal disks with grid-like surfaces, one on the floor and one on the ceiling directly over the other Between the two disks, floating unsupported in the air, hung a cloud of tiny sparks of light.

It looked like a swarm of minute golden bees, countless in number, and the swarm was lenticular in shape He said calmly, "Yes, Bradley, it is true. That is a tiny, self-sustaining universe, with its own suns, nebulae and worlds. Everything in it, down to the atoms which compose it, is infinitely smaller in scale than our own.

But it is a real universe, like our own. Dick had some fun with this idea in his short story The Trouble With Bubbles: The Worldcraft bubble glittered, catching the light Lora turned on the bubble. It glowed, winking into brilliance The idea is that our universe exists within a bubble, and multiple alternative universes are contained within their own distinct bubbles. In these other universes, the physical constants—and even the laws of physics—may differ drastically. Despite the theory's resemblance to science fiction, astronomers are now looking for physical evidence: Disc-shaped patterns in the cosmic background radiation left over from the Big Bang, which could indicate collisions with other universes.

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A Big Bang in a Little Room, by Zeeya Merali: Frontier physics and religion

Who is the New Jamestown Skeleton? Science Age of Humans. The Art of Secrets and Surveillance. At the Smithsonian Visit. Photos Submit to Our Contest. Photo of the Day. Subscribe Top Menu Current Issue. From end to end, the newly discovered gamma-ray bubbles extend 50, light-years, or roughly half of the Milky Way's diameter, as shown in this illustration. This pulsar captured in an image by the Chandra X-Ray grabbed attention for its eerie similarity to a human hand. One of the many mysteries baffling astronomers is how galaxies such as the Milky Way are able to form new stars at an unsustainable rate.

Why do only some parts of the Moon have a magnetic field? Recent science may indicate that it is a relic of an asteroid collision 4. The galaxy LEDA appears more or less like a rectangle, but no one knows why.