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The (Real) Big Bang Theory

Aprile 2019
La comunità scientifica riconosce in modo unanime che l’universo fu creato da una grande esplosione, il cosiddetto Big Bang. Nei secoli, i contributi di geni come Isaac Newton, Albert Einstein o Stephen Hawking ci hanno lasciato molte domande senza risposta, oltre a un infinito (è proprio il caso di dire) di possibilità da esplorare.

di Alex Phillips © The Institute of Art and Ideas

File audio:

Speaker: Sarah Davison (British accent)

How did it all begin? Did the universe have a creator? There are many scenarios for how the universe started, and how it then evolved. Some early scientists, such as 17th-century thinker Isaac Newton, thought that the universe was eternal: that infinity stretches forward and backward. Others thought that it was finite: that the universe must have a beginning and an end. And many, including early 20th century physicist Albert Einstein, formed halfway theories: that perhaps the universe has a beginning, but then it may go on forever. Sean Carroll is a research professor at Caltech, the California Institute of Technology. He outlines some of the influential ideas about the origins of the universe:

Sean Carroll (American accent):

If you were Isaac Newton, the universe just lasts forever. Time is absolute and it just goes on forever. Newton, actually, was a little bit worried about the implications of that. Newton thought that God would come in and rearrange the planets occasionally, ‘cause otherwise they would run off! But then once general relativity comes along with Einstein, you could show that if the universe is full of stuff it’s either expanding or contracting, so there was probably a crunch or a bang – there was a beginning or an end. And one of the first people to take this seriously was a priest, George Lemaître, and he developed this theory, which we would now call the Big Bang; maybe the universe, contra Newton, had a start. And you know who loved this idea? The Pope.

THE GOD QUESTION

A beginning suggested the need for a creator – a God. Obviously, the Pope liked this idea, but it proved problematic for scientists, not least those working in communist Russia in the 1960s! In Britain, however, a young physicist called Stephen Hawking had an idea. From the 1980s on, Hawking produced a series of books that, accompanied by his website and public lectures, attempted to make the universe accessible to all of us. He explained some of the debates going on in an incredibly speculative field. He believed that just because the universe began did not mean that a creator was essential to start the process. He thought that the universe could start itself and then be self-generating, as Carroll explained:

Sean Carroll:

One of the wonderful consequences of this way of thinking that Stephen Hawking pursued is that even a universe with a beginning can be fully self-contained. Just because it has a first moment in time, [it] does not require anything outside to push the button and get it started.

A NEW BEGINNING

Hawking and the mathematician Roger Penrose came up with a theory based on Einstein and Lemaître’s ideas. They suggested that the universe – space and time together – did originate in a Big Bang, when the whole universe and everything in it were compressed into a single point of infinite density. This they called a ‘space-time singularity’, an instant when the laws of conventional physics (including those of general relativity) broke down. The universe, they argued, evolved from the Big Bang completely independently of what it was like before. But, Carroll said, Hawking’s scenario was just one of many theories, with about a ten per cent probability of being correct:

Sean Carroll:

The Big Bang model of cosmology is simply the statement that fourteen billion years ago the universe was in a hot, dense state, it expanded and cooled and went from being very smooth to relatively lumpy, with all these stars and galaxies and so forth. It is true. There is no point in doubting the Big Bang model. But if you say, “Well, what happened at the very beginning, exactly what was the universe doing from one minute after to fourteen billion years after [that]?” That first minute is up for grabs.

BLOW UP!

Roger Penrose has since revised his ideas about the origins of the universe. He now says the problem is with the accepted idea that just after the Big Bang the universe inflated at an incredibly fast rate. Penrose finds this difficult to believe, and has come up with an alternative theory, as he explained:

Roger Penrose (English accent):

You think of a number: one fraction … the bottom denominator is a number which has thirty-two digits. And [in] that ridiculously small fraction of a second the universe was supposed to have expanded far more rapidly than anything that we’re aware of now. It’s called an ‘exponential expansion’ and it is very much part of conventional cosmology – that’s the weak point of conventional cosmology. Now to me, that [what] we think of as the Big Bang was the continuation of something, what I call an ‘aeon’. And these went on as far [back], as the theory goes, indefinitely.

PARALLEL UNIVERSES

A popular theory is one of multiverses; that as expansion occurred, many universes were created. This hypothesis gave rise to the idea of parallel universes, where, for example, another version of ourselves might enact those decisions that we did not make! This seems like a subject for science fiction, but Penrose argues that it is worth taking seriously:

Roger Penrose:

When people say “multiverse” you’re thinking [of] what you might call parallel, stacked up next to each other, but the point of view that I’m putting forward is sequential, so they’re one after the other. And they’re really very different. Now there are reasons that some people put forward which are worth taking seriously: that there might be parallel universes. Now I think these are arguments one needs to look at and take seriously.

FLEXITIME

Sean Carroll has also formulated his own ambitious theory of the origins of the universe! It questions the way we understand time:

Sean Carroll:

There is something very profound about the nature of time in our observable universe, namely that it has a direction. The reason why you remember yesterday and not tomorrow is ultimately because of what conditions were like at the Big Bang. That’s what set up the arrow of time. So my favourite view of that is that there is a much larger universe that we don’t see, that our little universe is a tiny little part of the whole picture and the whole picture is actually symmetric, that there are people in our past who think that we are in their past, that time runs in the opposite direction for them.  



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Explains

General relativity. Space and time together. [La teoria de] la relatività generale. Lo spazio e il tempo insieme. Formulata da Albert Einstein nel 1916, questa teoria stabilisce che il tempo e lo spazio formano un continuo, e che i corpi celesti con una grande massa (soprattutto pianeti, stelle e buchi neri) formano una curvatura in questo continuo. La forza di gravità, quindi, è il fenomeno che deriva dalla suddetta curvatura dello spazio e del tempo. Maggiore la massa di un corpo, maggiore è la forza di gravità che questo esercita su altri oggetti, i quali sono attratti verso di esso a causa della curvatura nel continuo spazio-temporale. Le teorie di Einstein rappresentarono una vera e propria rivoluzione per il concetto stesso dell’universo, e aprirono infinite possibilità speculative. Per esempio, se lo spazio-tempo si può curvare, esiste la possibilità che si chiuda in sé stesso (un fenomeno chiamato wormhole, letteralmente ‘buco di verme’), e ciò permetterebbe di viaggiare all’indietro nel tempo. La teoria della relatività generale completa la teoria della relatività ristretta, formulata da Einstein dieci anni prima, che stabilisce che il tempo non sia una costante, bensì relativo alla velocità con la quale l’osservatore si sposta.