What happened in the first moments after the Big Bang? When did the first particles of matter form, and when did the fundamental forces emerge? These are questions for which there are many speculations and theories, but no direct observations at earliest times.
The reason: the first 380,000 years of the 13.8-billion-year-old universe lie behind an impenetrable curtain of radiation and matter that prevents us from seeing into this past. The first directly observable signals at present are from the Cosmic Microwave Background (CMB), that was discovered in 1964. Signals from earlier times, such as visible light, micro or The authors now suggest, however, there can be a way to see behind this curtain. In analogy to the many supernovae observed at present times, there must have been violent bursts of energy in the early moments of the cosmos – whether from the formation of so-called “baby universes,” or other Big Bang-like explosive events such as the formation of supermassive primordial black holes. These bursts could emit highly penetrating particles which could lead to observable signals.
They propose three potential signal pathways. Neutrinos play a key role in two of them. These particles are released during all astrophysical events that take place at very high energies. Moreover, they are highly penetrating and could escape to us.
Possible evidence of weak X-rays
Based on the role high-energy neutrinos play in violent environments, it is plausible that they are also produced during bursts in the early universe. Due to their properties, they could slip through the cosmic curtain, but would lose most of their energy on their way to Earth. During this process, positrons, i.e. antimatter, could be produced.
The positron is the antiparticle of the electron. When the two meet, they annihilate each other and energy in the form of photons is released. This could be detected as weak X-rays. They are redshifted, i.e. moving away from us, lowered in energy and arrive as soft extragalactic x-rays. This gives a possibly detectable signal, with a characteristic peak at a certain low energy. The peak could be observed in the soft x-ray sky. It may have hitherto eluded detection by x-ray surveys, since the expected, very weak signal is lost in the noise. To detect it scientists would need long observation times that generate large amounts of data.
A new low-energy neutrino background
The authors also investigate a second cosmic neutrino signature: an unexpectedly high low-energy neutrino background in today's universe. In the case of the early bursts, we could also expect the direct production of greatly redshifted low-energy neutrinos. Their origin would be moving away closely at the speed of light. This means, the neutrinos do not interact with their environment and simply arrive to the present time with very low energy. As opposed to the soft x-rays, the technology of their detection is still an open question.
Hot spots in the microwave background
A third possibility for observation of the bursts would be the existence of “hot spots” on the CMB, in particular small regions with an out-of-equilibrium spectrum. This has been the subject of intensive research for decades, including the European “Planck” mission, in which the MPI for Astrophysics played a major role. The big challenge here is to identify these very small spots and determine their spectrum. This will require excellent angular resolution and advanced statistical methods.
The two authors hope that their theoretical work will stimulate developments necessary to observe these signals – messengers directly from the early universe. Their detection would pave the way to seeing more deeply into the Big Bang. It would shed light into the beginning of the universe and open up new chapters in observational cosmology.