Early universe may have been dominated by bobbing waves

July 24, 2012 by Lisa Zyga, Phys.org feature
Oscillon simulations show that inflation may have been immediately followed by an oscillon-dominated phase of the early universe. Image credit: Amin, et al. ©2012 American Physical Society

(Phys.org) -- Localized waves that bob up and down without dissipating their energy, called “oscillons,” may have dominated the early universe shortly after inflation. A collaboration of physicists from MIT, Yale University, and Stanford University has discovered that copious amounts of oscillons arise in simulations based on several realistic inflationary models and could have caused novel gravitational effects in the early universe, although it is unclear whether the effects could be directly observed today.

The physicists have published their paper on the possibility of oscillons existing after inflation in a recent issue of Physical Review Letters.

As the scientists explain in their paper, oscillons are massive, long-lived excitations of a scalar field that are localized, i.e., they do not dissipate like ripples produced by dropping a pebble in a calm pond. Instead, an oscillon switches between being a hill and a crater, alternately rising above and sinking below the spatially uniform state of the field. In previous experiments, scientists have created oscillons by vertically vibrating a plate with a sufficiently thick layer of granular particles. As long as they’re not disturbed, oscillons will continue to move up and down for hundreds of thousands of oscillations.

Oscillon-dominated universe

In the new study, the physicists used simulations to investigate the requirements needed for oscillons to form just after inflation, the period of rapid expansion that occurred from 10-36 to 10-33 seconds after the Big Bang. In a class of inflationary models that include some string models and supergravity-inspired models, the scientists found that inflation is followed by self-resonance that in turn generates large numbers of oscillons.

“At the end of inflation, the inflaton (the agent responsible for inflation) is oscillating up and down, in sync throughout the universe,” coauthor Mustafa Amin at MIT told Phys.org, speaking on behalf of his coauthors Richard Easther, Raphael Flauger, and Hal Finkel at Yale University (now at the University of Auckland in New Zealand, the Princeton Institute for Advanced Studies, and Argonne National Laboratory, respectively), and Mark Hertzberg at Stanford University.

“However, this synchronous, homogeneous state cannot be maintained for long,” he explained. “It is unstable and quickly fragments into an inhomogeneous, clumpy state. The rapid transfer of energy from the synchronous to the clumpy state is what we call self-resonance. We refer to it as self-resonance because the energy is shuttled from the synchronous, homogeneous state to the clumpy state of the inflation field itself. Interestingly, the mathematics describing this transfer of energy is identical to that describing a child pumping a swing. The child's pumping plays the role of the homogenous, oscillating inflaton whereas the arc of the swing is related to the energy in the clumpy state.”

According to the simulations, the oscillons in the would have lived long enough for the universe to grow by a factor of 100 or more, under the assumption that the energy transfer to other particles is suppressed. This period could alter our view of what the universe looked like between the end of inflation and the beginning of radiation domination.

“In many models of particle production after inflation, the inflaton rapidly transfers its energy to a turbulent sea of intermediary particles, which eventually decay into the particles we know and love,” Amin said. “Our work shows that, even if the energy transfer to other particles is suppressed, the inflaton can fragment rapidly into localized, coherent structures called oscillons. These oscillons are so ubiquitous that they can dominate the energy density of the universe at that time, which is definitely novel! In terms of altering our basic picture of the early universe, the presence of oscillons increases the clumpiness of the universe on small scales at the end of inflation. Their presence might also alter how the energy from the inflaton is eventually transferred to other particles, and how long it takes to do so. However, to make concrete statements about this and its impact on astrophysics in general, we need to study more realistic models including many more types of particles and gravitational interactions between oscillons.”

Amin explained that, although these oscillons would have been much too small to see and very short-lived, the sheer number of them would have enabled them to have a significant impact on the early universe.

“In the class of models we have considered, the oscillons are smaller than the Hubble horizon at the time of production,” he said. “The Hubble horizon is a measure of the typical distance light can travel while the universe doubles in size, and can be smaller than the size of an atom. Although small compared to human scales, oscillons are large in the following sense. Oscillons can be thought of as a bag of a large number of inflaton particles. This bag of individual particles can be extremely heavy compared to the heaviest known particles. Although exact numbers of oscillons produced depend on the details of the models used, enough of them can be produced to dominate the energy density of the universe at that time. Their lifetime is a tiny fraction of a second. However, even in this tiny fraction of a second, the universe doubles in size many times over! So as far as this doubling timescale of the universe is concerned, they live extremely long and can have a strong impact on the dynamics within the universe at that time. Regarding whether they move in unison, our simulations show that most of the oscillons do not move much once they are formed (as we mentioned before, they are rather heavy), and the distance between them increases as the universe expands. However, if the gravitational force between them is included, they might clump together to form clusters of oscillons.”

Oscillons are localized, time-periodic, extremely long-lived excitations of scalar fields with non-linear interactions. The animation above is a 2D slice through the center of an oscillon. Video credit: Amin, et al.

Gravitational effects

The physicists calculated that an oscillon-dominated universe would affect the density fluctuations that appeared around that time which later led to the formation of galaxies, stars, and planets. They found that the abundant oscillons would have enhanced the frequency spectrum of the early inhomogeneities, or primordial power spectrum, on very small scales. This enhancement, in turn, could have possibly led to novel gravitational effects at that time.

“When gravity is included, oscillons attract each other, possibly forming bound structures and leading to structure formation not too different than what happens in the contemporary universe (except this happens on a very tiny length scale and very early on in the history of the universe),” Amin said. “In addition, oscillons can merge, fragment or scatter off each other in complex ways when they come close to each other. Another (even more speculative) possibility is that some of these objects could combine and collapse to form primordial black holes, which could then quickly evaporate. All of these possibilities need further work, and will keep us busy.”

He added that the presence of oscillons would probably not have had any effect on present-day observations, since oscillons existed on scales much smaller than those that contribute to large-scale structure formation

“It is unclear whether these primordial oscillons will have an effect today,” Amin said. “We have not fully understood their implications yet, especially when other particles and gravity is included in the simulations.”

Amin and his coauthors are not the only scientists investigating the possible existence of oscillons shortly after inflation. Other researchers, including those at Dartmouth College and the University of Sussex, have also found that oscillons can be produced easily in different early universe processes. But they still have many unanswered questions to investigate.

“Some of us are working on trying to understand how robust these oscillons are when new particles are included in the simulations, which can bleed energy from the oscillons,” Amin said. “Some questions we are interested in include: (1) Does the presence of oscillons delay the rate at which the inflation transfers its energy to other particles? (2) How do oscillons clump in the presence of gravity? Do they form clusters? Can they collapse to form primordial black holes? (3) We are also interested in studying what happens during collisions of oscillons. Such interactions will be important to understand the ultimate fate of a collection of oscillons produced in the early universe.”

Explore further: Could primordial black holes be dark matter?

More information: Mustafa A. Amin, et al. “Oscillons after Inflation.” PRL 108, 241302 (2012). DOI: 10.1103/PhysRevLett.108.241302

Journal reference: Physical Review Letters search and more info website


Related Stories

Could primordial black holes be dark matter?

September 21, 2011

(PhysOrg.com) -- “We know that about 25% of the matter in the universe is dark matter, but we don’t know what it is,” Michael Kesden tells PhysOrg.com. “There are a number of different theories about what ...

Physicists investigate lower dimensions of the universe

March 18, 2011

(PhysOrg.com) -- Several speculative theories in physics involve extra dimensions beyond our well-known four (which are broken down into three dimensions of space and one of time). Some theories have suggested 5, 10, 26, ...

Recommended for you


Adjust slider to filter visible comments by rank

Display comments: newest first

1 / 5 (1) Jul 24, 2012
the oscillons in the early universe would have lived long enough for the universe to grow by a factor of 100 or more
But the inflation theory considers an expansion in the range 10E40, thus leaving all primordial artefacts bellow detection limit of any contemporary technology.
1 / 5 (4) Jul 24, 2012
Why would an expansion in the range of 10^40 in and of itself make primordial artifacts be below detectable limits? If the primordial particle is stable enough to have survived, it should theoretically be available to be detected.
1 / 5 (1) Jul 24, 2012
Beyond the local light cone.
5 / 5 (5) Jul 24, 2012
A preprint of the PRL paper is available here: http://arxiv.org/abs/1106.3335
1.3 / 5 (4) Jul 24, 2012
"Beyond the local light cone."

Is it not possible that some primordial artifacts traveled along during inflation with the the normal matter we see? Why must it have only existed then,,, what I was asking: Assuming it was/is stable enough to have not decayed out of existence, could it not be detected today?
1 / 5 (2) Jul 24, 2012
Periodic oscillations have a tendency to synchronize or self organize in one or more precise patterns, as described by Art Winfree (biomathematician, MacArthur prize) circa 1967. See Strogatz and Stewart, "Coupled Oscillators and Biological Synchronization," Scientific American, Dec. 1993 (copy available on internet at a math.oregonstate.edu site).

Assuming the Winfree notion applies--which is a reach, admittedly, though string theory is based on vibrating strings so it might be plausible-- this self-organization should be true of the homogenous state (which is "synchronous," according to the article)and the clumpy state (which the article does not specifically describe as synchronous or otherwise).

Using the "arc of the swing" mental model as a proxy for the clumpy state, the swing arc is also a periodic oscillation, and so might self-organize with other swing arcs based on Winfree's theory, if it may be so extensively generalized.
3 / 5 (4) Jul 24, 2012
Why would an expansion in the range of 10^40 in and of itself make primordial artifacts be below detectable limits?

Beyond the local light cone.

Kinda, but not because it's too far away. It should actually be everywhere, like the CMBR.

The reason it should be beyond detection is because it would be stretched (by expansion) so much that we would not be able to detect any variation above the plank limit within the visible part of the Universe. In other words, it's all around us, but it looks so uniform that we can't tell it is there. In escence, you would need to be able to compare states between parts of the universe which are now seperated by more than 13.7 billion light years, in order to see a detectable difference.
1 / 5 (2) Jul 24, 2012
Yes, and dark energy can be an inflationary aftershock; i'm amazed at the negative reaction that astrophysicists and mathematicians that I've brought this idea up to have had to this idea!

We'll see . . . !
5 / 5 (3) Jul 25, 2012
flashgordon, I am not surprised at all.

What does "inflationary aftershock" even mean, it sounds like tow words randomly positioned close together? And how do you go from there to predict the magnitude of the cosmological constant dark energy seems to be?

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

Click here to reset your password.
Sign in to get notified via email when new comments are made.