Study outlines new proposal for probing the primordial universe

Harvard study proposes new way to probe universe before the Big Bang
Credit: Harvard University

Most everybody is familiar with the Big Bang—the notion that an impossibly hot, dense universe exploded into the one we know today. But what do we know about what came before?

In the quest to resolve several puzzles discovered in the initial condition of the Big Bang, scientists have developed a number of theories to describe the , the most successful of which—known as —describes how the dramatically expanded in size in a fleeting fraction of a second right before the Big Bang.

But as successful as the inflationary theory has been, controversies have led to active debates over the years.

Some researchers have developed very different theories to explain the same that have supported the inflationary theory so far. In some of these theories, the primordial universe was contracting instead of expanding, and the Big Bang was thus a part of a Big Bounce.

Some researchers—including Avi Loeb, the Frank B. Baird, Jr. Professor of Science and chair of the Astronomy Department—have raised concerns about the theory, suggesting that its seemingly endless adaptability makes it all but impossible to test.

"The current situation for inflation is that it's such a flexible idea … it cannot be falsified experimentally," Loeb said. "No matter what result of the observable people set out to measure would turn out to be, there are always some models of inflation that can explain it." Therefore, experiments can only help to nail down some model details within the framework of the inflationary theory, but cannot test the validity of the framework itself. However, falsifiability should be a hallmark of any scientific theory.

That's where Xingang Chen comes in.

A senior lecturer in astronomy, Chen and his collaborators for many years have been developing the idea of using something he called a "primordial standard clock" as a probe of the primordial universe. Together with Loeb and Zhong-Zhi Xianyu, a postdoctoral researcher in the Physics Department, Chen applied this idea to the noninflationary theories after he learned about an intense debate in 2017 that questioned whether inflationary theories make any predictions at all. In a paper published as an Editor's Suggestion in Physical Review Letters, the team laid out a method that may be used to falsify the inflationary theory experimentally.

In an effort to find some characteristic that can separate inflation from other theories, the team began by identifying the defining property of the various theories—the evolutionary history of the size of the primordial universe. "For example, during inflation, by definition the size of the universe grows exponentially," Xianyu said. "In some alternative theories, the size of the universe contracts—in some very slowly and in some very fast.

"The conventional observables people have proposed so far have trouble distinguishing the different theories because these observables are not directly related to this property," he continued. "So we wanted to find what the observables are that can be linked to that defining property."

The signals generated by the primordial standard clock can serve this purpose.

That clock, Chen said, is any type of massively heavy elementary particle in the energetic primordial universe. Such particles should exist in any theory, and they oscillate at some regular frequency, much like the swaying of a clock's pendulum.

The primordial universe was not entirely uniform. Quantum fluctuations became the seeds of the large-scale structure of today's universe and one key source of information physicists rely on to learn about what happened before the Big Bang. The theory outlined by Chen suggests that ticks of the standard clock generated signals that were imprinted into the structure of those fluctuations. And because standard clocks in different primordial universes would leave different patterns of signals, Chen said, they may be able to determine which theory of the primordial universe is most accurate.

"If we imagine all the information we learned so far about what happened before the Big Bang is in a roll of film frames, then the standard clock tells us how these frames should be played," Chen explained. "Without any clock information, we do not know if the film should be played forward or backward, fast or slow—just like we are not sure if the primordial universe was inflating or contracting, and how fast it did that. This is where the problem lies. The standard clock put time stamps on each of these frames when the film was shot before the Big Bang, and tells us what this film is about."

The team calculated how these standard clock signals should look in noninflationary theories, and suggested how to search for them in astrophysical observations. "If a pattern of signals representing a contracting universe were found," Xianyu said, "it would falsify the entire inflationary , regardless of what detailed models one constructs."

The success of this idea lies in experimentation. "These signals will be very subtle to detect," Chen said. "Our proposal is that there should be some kind of massive fields that have generated these imprints and we computed their patterns, but we don't know how large the overall amplitude of these signals is. It may be that they are very faint and very hard to detect, so that means we will have to search in many different places.

"The cosmic microwave background radiation is one place," he continued. "The distribution of galaxies is another. We have already started to search for these signals and there are some interesting candidates already, but we still need more data."


Explore further

What happened before the Big Bang?

More information: Xingang Chen et al. Unique Fingerprints of Alternatives to Inflation in the Primordial Power Spectrum, Physical Review Letters (2019). DOI: 10.1103/PhysRevLett.122.121301
Journal information: Physical Review Letters

Provided by Harvard University

This story is published courtesy of the Harvard Gazette, Harvard University's official newspaper. For additional university news, visit Harvard.edu.

Citation: Study outlines new proposal for probing the primordial universe (2019, April 19) retrieved 25 May 2019 from https://phys.org/news/2019-04-outlines-probing-primordial-universe.html
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.
227 shares

Feedback to editors

User comments

Apr 19, 2019
This is new, inflation occurred *Before* the Big Bang?

Apparently the folks over at Wikipedia haven't heard the news:

"In physical cosmology, cosmic inflation, cosmological inflation, or just inflation, is a theory of exponential expansion of space in the early universe. The inflationary epoch lasted from 10^−36 seconds after the conjectured Big Bang singularity to some time between 10^−33 and 10^−32 seconds after the singularity. Following the inflationary period, the universe continues to expand, but at a less rapid rate."

https://en.wikipe...smology)

Note that there is no direct evidence of any kind for inflation and the inflation conjecture includes no theory of why it started, how it started, or how and why it stopped. It is about as plausible as String Theory, multiple universe theories and philosophical musings loosely presented in scientific guise. Have faith...it is all that holds these speculations aloft...

Apr 19, 2019
Just so we're clear, if all the observations militate toward one particular theory, as opposed to many that can use inflation, that would be pretty indicative. It's unfortunate that the writer of this article seems not to know that.

Apr 19, 2019
In physical cosmology, cosmic inflation, cosmological inflation, or just inflation, is a theory of exponential expansion of space in the early universe. The inflationary epoch lasted from 10^−36 seconds after the conjectured Big Bang singularity to some time between 10^−33 and 10^−32 seconds after the singularity. Following the inflationary period, the universe continues to expand, but at a less rapid rate.
I've had issues with that since it was written. First there's the whole "singularity" thing, then there's the part about the Big Bang happening before inflation. I don't think they quite get it.

https://en.wikipe...smology)

Apr 19, 2019
Note that there is no direct evidence of any kind for inflation and the inflation conjecture includes no theory of why it started, how it started, or how and why it stopped. It is about as plausible as String Theory, multiple universe theories and philosophical musings loosely presented in scientific guise. Have faith...it is all that holds these speculations aloft...
I don't think you've examined the evidence. There's quite a lot of evidence, actually, and the fact you don't know about it tells me you're bloviating.

I'll be happy to post some examples, but first you'll have to convince me you're not just another anti-inflation troll.

Apr 19, 2019
And unfortunately, looking at the paper, they continue to assume a la Wikipedia that inflation happened *after* the Big Bang, not *before.* There won't be anything to detect from before all the matter and energy appeared in the cosmos. Certainly not from atoms. Another meaningless Chinese scientific troll job. Let it sit a few years and let's see how many citations it gets.

Apr 19, 2019
"The current situation for inflation is that it's such a flexible idea … it cannot be falsified experimentally," Loeb said.

That is an opinion, inflation has become the consensus conclusion since it passed 5 out of 6 early tests. The paper admits as much, but take issue with that each test is not falsifying all the contender theories.

Moreover, the 2018 Planck data release combined new methods and new data from other telescopes and could test and constrain what type of inflation - slow roll or chaotic - we see. We are doing normal science on it, and while contender theories cannot be rejected inflation has been doing increasingly well and they increasingly badly, which is a good sign in these situations.

That said, more tests are welcome!

- tbctd -

Apr 19, 2019
- ctd -

"That clock, Chen said, is any type of massively heavy elementary particle in the energetic primordial universe."

That would at the very least be the inflation particles of the inflation field. But the inflation era was exceedingly cold and dilute, so the interaction between its particles and its virtual particles would be subtle.

And unfortunately, looking at the paper, they continue to assume a la Wikipedia that inflation happened *after* the Big Bang, not *before.* ... Another meaningless Chinese scientific troll job. Let it sit a few years and let's see how many citations it gets.


I don't see that they do, they chose a technical parametrization but claim that the result is independent (I did not check). The usual cosmological scale factor a(t) in terms of comoving volumes of the observable universe goes to zero and they wanted to go into the far past while referring back to it. 2nd author Abraham Loeb is a merited (though fanciful) Israeli-US scientist.

Apr 19, 2019
Let me add that I find it refreshing that others are nitpicking the confusion that lingers, not so much in the papers (or at least the non-fringe ones) but in the university press releases [oy!] as here and popular press to boot.

A nice resource is both particle physicist Matt Strassler, say https://profmatts...niverse/ and https://profmatts...niverse/hot-big-bang/ , and cosmologist Ethan Siegel, say https://www.forbe...03de55df and https://www.forbe...8cd17d81 . I have taken to define Big Bang as the Hot Big Bang, with the most likely (i,e unconstrained) embedding suggested by Planck 2018: eternal slow roll process, inflation stopped locally.

Apr 19, 2019
"That clock, Chen said, is any type of massively heavy elementary particle in the energetic primordial universe. Such particles should exist in any theory, and they oscillate at some regular frequency, much like the swaying of a clock's pendulum."

Should they exist in any theory? Restricted simply to any inflationary theory, I don't find it parsimonious to posit the necessary existence of any particle whatsoever b e f or e the fractionation of the 4 fields, and the -- a f t e r w a r d -- crystallization out of the Standard Model particles plus any predictably transient exotica.

Chen himself is simply injecting a new claim, with its own falsifiability issues. I take inflationary models as a given, but regard any sort of cosmogenic theory as likely metaphysical, not physical. Even Roger Penrose was self-effacing enough about his own scheme to persistently present it in his lectures along with a sort of 'hey, I got this nutty idea...' preamble.

Apr 19, 2019
If we believe that our World has started sometimes ago we are still in the position to decide which hypothesis, Lemaître's or Gamow's was closer to reality. There is an opinion that the problems in the standard cosmology could be solved by adjusting of details. Our suggestion is that we have to go back to the conceptions and use the observations accumulated since.
https://www.acade...osmology

Apr 20, 2019
Let a big bang be a being. Since it is a being and sleeping have a vision as dreams. Dreams create fluctuations due to feeling as fluctuations antimatter is generated and a big bang occurs in the being. Do you like the story.

Apr 20, 2019
Chen himself is simply injecting a new claim, with its own falsifiability issues.
You're correct here. But the reason is because the inflaton is lambda, the cosmological constant. It was when the physicists working on inflation realized that if they chose an appropriate value of cosmological constant in GRT they got inflation that they started thinking they were onto something.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more