New theory finds upcoming satellite mission will be able to detect more
than expected

Date:
January 12, 2022
Source:
Kavli Institute for the Physics and Mathematics of the Universe
Summary:
Researchers have theorized that in addition to the gravitational
waves originating from vacuum fluctuations during inflation, a
large amount of gravitational waves can be sourced by the quantum
vacuum fluctuations of additional fields during inflation.



FULL STORY ==========================================================================
The upcoming satellite experiment LiteBIRD is expected to probe the
physics of the very early Universe if the primordial inflation happened
at high energies.

But now, a new paper in Physical Review Letters shows it can also test inflationary scenarios operating at lower energies.


========================================================================== Cosmologists believe that in its very early stages, the Universe underwent
a very rapid expansion called "cosmic inflation." A success story of
this hypothesis is that even the simplest inflationary models are able
to accurately predict the inhomogeneous distribution of matter in the
Universe. During inflation, these vacuum fluctuations were stretched
to astronomical scales, becoming the source all the structure in the
Universe, including the Cosmic Microwave Background anisotropies,
distribution of dark matter and galaxies.

The same mechanism also produced gravitational waves. These propagating
ripples of space and time are important for understanding the physics
during the inflationary epoch. In general, detecting these gravitational
waves is considered determining the energy at which inflation took
place. It is also linked to how much the inflation field, or the energy
source of inflation, can change during inflation -- a relation referred
to as the "Lyth bound." The primordial gravitational waves generated
from vacuum are extremely weak, and are very difficult to detect, but
the Japanese-led LiteBIRD mission might be able to detect them via the polarization measurements of the Cosmic Microwave Background. Because
of this, understanding primordial gravitational waves theoretically
is gaining interest so any potential detection by LiteBIRD can be
interpreted. It is expected LiteBIRD will be able to detect primordial gravitational waves if inflation happened at sufficiently high energies.

Several inflationary models constructed in the framework of quantum
gravity often predict very low energy scale for inflation, and so
would be untestable by LiteBIRD. However, a new study by researchers,
including the Kavli Institute for the Physics and Mathematics of the
Universe (Kavli IPMU), has shown the opposite. The researchers argue
such scenarios of fundamental importance can be tested by LiteBIRD, if
they are accompanied by additional fields, sourcing gravitational waves.

The researchers suggest an idea, logically very different from the usual.



========================================================================== "Within our framework in addition to the gravitational waves originating
from vacuum fluctuations, a large amount of gravitational waves can be
sourced by the quantum vacuum fluctuations of additional fields during inflation. Due to this we were able to produce an observable amount of gravitational waves even if inflation takes place at lower energies.

"The quantum fluctuations of scalar fields during inflation are
typically small, and such induced gravitational waves are not relevant
in standard inflationary scenarios. However, if the fluctuations of the additional fields are enhanced, they can source a significant amount of gravitational waves," said paper author and Kavli IPMU Project Researcher Valeri Vardanyan.

Other researchers have been working on related ideas, but so far no
successful mechanism based on scalar fields alone had been found.

"The main problem is that when you generate gravitational waves from
enhanced fluctuations of additional fields, you also simultaneously
generate extra curvature fluctuations, which would make the Universe
appear more clumpy than it is in reality. We elegantly decoupled the
generation of the two types of fluctuations, and solved this problem,"
said Vardanyan.

In their paper, the researchers proposed a proof-of-concept based on
two scalar fields operating during inflation.

"Imagine a car with two engines, corresponding to the two fields of
our model.

One of the engines is connected to the wheels of the car, while the
other one is not. The first one is responsible for moving the car, and,
when on a muddy road, for generating all the traces on the road. These represent the seeds of structure in the Universe. The second engine is
only producing sound. This represents the gravitational waves, and does
not contribute to the movement of the car, or the generation of traces
on the road," said Vardanyan.

The team quantitatively demonstrated their mechanism works, and even
calculated the predictions of their model for the upcoming LiteBIRD
mission (image).

Details of their study were published on December 15 in Physical Review Letters.

========================================================================== Story Source: Materials provided by Kavli_Institute_for_the_Physics_and_Mathematics_of_the Universe. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Yi-Fu Cai, Jie Jiang, Misao Sasaki, Valeri Vardanyan, Zihan
Zhou. Beating
the Lyth Bound by Parametric Resonance during Inflation. Physical
Review Letters, 2021; 127 (25) DOI: 10.1103/PhysRevLett.127.251301 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220112121542.htm
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