GroundBIRD

Inflation

Cosmologists need a period of extreme inflation in the first 10^-32 seconds after the birth of the Universe in order for their model to still be consistent with observations. The Universe would then have grown by a factor of 10²⁷ in that short period of time.

Horizon problem

One of the problems encountered by the standard model of cosmology is that on the larger scale the Universe has the same temperature everywhere. No matter which direction we look, we see uniform radiation that an object of approximately 2.7 Kelvin would emit, so 2.7 degrees above absolute zero (-273.15 °C). In this so-called cosmic microwave background (CMB) we measure spatial variations in the order of one thousandth of a percent.

The problem is that areas on one side of the Universe have never been able to communicate with areas on the other side, because the speed of light is too low and the distances are too great. So how can their temperatures match so closely? Cosmologists call this the horizon problem. Only if extreme inflation occurred shortly after the birth of the Universe, could areas that were previously in contact with each other now be so far apart.

Flatness problem

The second problem with the standard model of cosmology is that the Universe appears to be exactly flat, with precisely the right density of matter and energy. By this we mean that the Universe is not spherical, where the angles of a triangle add up to more than 180 degrees, nor is it concave. Astronomers have drawn a triangle between the Earth and two points in the sky, and based on the CMB they can see with an accuracy of only 0.2% that the angles add up neatly to 180 degrees, as on a sheet of paper.

The problem here is that this flatness seems like an absurd coincidence, an impossible stroke of luck for humanity. If the density were slightly higher or lower, the Universe would collapse shortly after the Big Bang or fly apart before stars, planets and humans could form. The density, and therefore flatness, of the Universe must have a critical value so ridiculously precise that cosmologists speak of an impossible coincidence. The margin is an inconceivable 1 in 10⁶².

Inflation solves this problem too. The Universe may well have had a convex or concave shape right after the birth of the Universe, as long as this was followed by extreme inflation in which any curvature was eliminated. Think of an ant on a deflated balloon that clearly perceives ripples. When the balloon is inflated, the ant will perceive it as a flat surface.

Magnetic Monopole Problem

Theories of particle physics predict that large amounts of magnetic monopoles were produced right after the birth of the Universe. These are heavy particles with only one magnetic pole, contrary to the bipolar magnets we all know. Still, we have never found a monopole no matter how hard we look. Inflation solves this discrepancy between theoretical prediction and observation. By exponentially expanding space, inflation spreads out the created monopoles to such an extent that the chance of one ending up in our vicinity is practically zero.

Structure Formation Problem

Finally, there is a problem with the structure of the Universe, including all the galaxies, planetary systems and ourselves: it shouldn’t exist. If the Universe started out as a perfectly uniform chunk of energy, then how could there have been seeds for any type of structure? Also here inflation comes to the rescue.

At the smallest scales, we enter the realm of quantum mechanics, where there is no such thing as smoothness. The energy of each location in the Universe is constantly fluctuating. At the moment inflation took flight, those energy variations suddenly surged into macroscopic quantities and settled into the seeds for the structure of the Universe as we know it.

Cosmic microwave background

To find evidence that inflation actually took place, GroundBIRD is conducting measurements of the CMB. This radiation contains signatures from the period following the Big Bang. During the first few hundred thousand years, radiation could not move freely through the Universe because of the fog created by an excess of free-flying electrons and protons. After 380,000 years, the Universe had cooled sufficiently for electrons to bind to atomic nuclei, causing the fog to quickly dissipate.

From that moment onwards, radiation has been racing uninterrupted through the Universe in all directions. GroundBIRD therefore captures radiation from all directions that has been travelling throughout the full age of the Universe – 13.8 billion years. We call this the cosmic microwave background (CMB). It seems to come from a spherical shell around us with a radius of billions of lightyears. But in fact, it was emitted from a shell much closer to us, at wavelengths in the near-infrared. The expansion of the Universe has stretched it to the wavelengths at which we observe it: microwaves.

Polarisation

The CMB is largely unpolarised. This means that all photons oscillate in random directions as they travel towards us. The same goes for sunlight. However, an external factor can give photons a slight preference for a particular oscillation direction. In the case of sunlight, it is the Earth’s atmosphere that influences the photons to oscillate in a certain direction. In other words, it becomes polarised. In the case of the CMB, the tiny irregularities that were present in the Universe are the factor that add a slight polarisation.

E-modes and B-modes

Depending on the origin of the irregularities, it could create two types of polarization in the CMB: E-modes and B-modes. While E-modes are created by both density fluctuations and gravitational waves, B-modes are created only by distortions of space due to gravitational waves. So B-modes in the CMB polarization show footprints of gravitational waves produced during cosmic inflation. By measuring the strength of B-mode, we can sense the energy scale of inflation and the underlying physics.

Since gravitational waves are tiny spacetime distortions, the strength of B-mode polarization is also expected to be very small. Estimated to be a fluctuation in the order of 10^-9 K at its largest on top of the overall 2.7 K, the B-mode polarization has not been observed yet, despite many observational attempts so far.