There was entropy before the universe began
Was there a universe before the Big Bang?
It sounds bold: Our universe did not begin with the Big Bang, it is just a second infusion of an older cosmos. Or the third or the fourth or the umpteenth. She can't say for sure either, but she's pretty sure that the story didn't just begin 13.8 billion years ago, as can be read in textbooks. "The model, according to which the Big Bang is the beginning of everything, poses a number of problems," says Anna Ijjas from the Max Planck Institute for Gravitational Physics in Hanover. "So I'm working on a better explanation." One could say that the Big Bang instead of the Big Bang. A cyclical universe that expands and eventually contracts, then expands again, and so on.
"Let's take today's universe, which is 10 to the 28 centimeters, a number with 29 digits," says the researcher. That will expand for about 100 billion years, then it will shrink again. First the size of the Milky Way, then our solar system, our earth, a football, an influenza virus. Until it is about 10 to the minus 25 centimeters - a number that only changes 25 places after the decimal point. And then it starts again.
These extremes are not unusual for a cosmologist. Ijjas, born 1985 in Hungary and came to Germany as a teenager, initially devoted himself to the philosophy of physics and wrote a dissertation on it in 2010 at the Ludwig Maximilians University in Munich. She completed her next doctorate in theoretical physics in 2014 at the Humboldt University in Berlin. This was followed by stays at Princeton, Columbia University and Harvard. Already during his second doctorate, Ijjas dealt with the Big Bang and began to doubt whether the history of the universe actually played out as it is formulated in standard works. Above all, the concept of cosmic inflation appears problematic to her.
The term describes a faster than light expansion of space immediately after the Big Bang, which lasted for a few fractions of a second. "The idea was developed in the 1980s to explain why the universe as a whole is so uniform and simple, downright boring," says Ijjas. If the universe were to expand slowly, the random quantum fluctuations of the Big Bang should form areas of different energy densities, which can still be recognized today in the distribution of matter.
But the astrophysicists see nothing of that. "The inflation theory was invented to explain that," says Ijjas. "The rapid expansion in the early phase is intended to iron away the rough unevenness of the Big Bang so that the universe assumes the simple shape as we know it."
But the trick has its pitfalls. For Ijjas, originally set up to clear up inconsistencies in the inflation theory, they were no longer acceptable when the data from the Planck satellite of the European Space Agency (ESA) were published in spring 2013. The experiment measured the cosmic background radiation with unprecedented accuracy and provided a picture of the young universe that is too simple for many variants of the inflation theory. In fact, no trace of primordial gravitational waves was found. These tiny and uniform fluctuations in space-time are intended to be a clear indication of the inflationary expansion. "The simplest models that can be found in textbooks were eliminated in one fell swoop," recalls the researcher.
Advocate of the original impact
Shortly after this discovery, Ijjas worked as a postdoc at Princeton with Paul Steinhardt, an authority on cosmology. He helped develop the theory of inflation, but turned away from it in the early 2000s and became an advocate of the Urprall model. "He encouraged me not to have too much respect for alleged solutions and to be open to alternative ideas." Whereupon she also ends up at the Urprall and tries to describe the concept with formulas.
The annoying problem of the singularity - to put it simply: the part where classical physics fails, inside black holes or at the Big Bang - you could get around, she says, and relies on a modification of Einstein's field equations. According to this, a contracting universe would start expanding again just in time before it leaves the field of describable physics. Similar models have been discussed under the term "big bounce" for some time.
Lots of criticism
The reactions of the professional community to Ijjas ’work were predictable. "There was a lot of criticism, including personal attacks, I would damage the community. But that's normal when you break new ground. You have to endure that," says Ijjas. She hardly sees any connection with the fact that she is a woman. "It was more related to my youth." Some of the sharp counter-speeches from the past would no longer be made today and instead rely on professional exchange.
Unusual theses require strong evidence, Ijjas knows that too. She has already made progress in the mathematical description of the Urprall, and in one or two years she would like to present a computer simulation. "If she is successful with it, her model will be taken more seriously," says Torsten Enßlin from the Max Planck Institute for Astrophysics in Garching, leading researcher on the Planck mission and not involved in Ijjas' work. "Many cosmologists are now more open to alternatives to inflation theory," he confirms and warns against drawing quick conclusions. "Both theories, the Big Bang and the Urprall, make daring assumptions about forms of energy that we do not yet know and extrapolate from today's conditions into really wild areas of the cosmos. It is not unlikely that something will go wrong."
The decisive factor will be to what extent astrophysical measurements confirm or refute assumptions. Although she is a theorist, Ijjas attaches great importance to close contact with experimenters. So she went to Hanover, where the satellite-based gravitational wave observatory Lisa is being planned. "Perhaps the most important experiment of the century!"
What would it mean for you if measurement data refuted the idea of the Urprall? "Of course, I would initially regret that. However, it is always our mistakes that provide us with new knowledge and point the way to a better explanation."
But it would be a shame, she adds. "In a cyclical universe something always happens, the cosmos recycles itself through the Urprall - that's exciting. In the Big Bang model, on the other hand, something is only going on for a few billion years, then everything falls apart and nothing happens for all eternity more. How boring. " (Ralf Nestler, January 19, 2020)
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