How can we observe the birth of the universe?

The origin of the universe: Looking back to the beginning

October 22, 2025

The image shows a representation of the cosmic microwave background, a kind of echo of the Big Bang. It is an oval with different colored dots. These show that there were fluctuations in temperature and energy densities in the universe from the beginning, from which the large-scale structures in the universe later emerged. — The cosmic microwave background is a snapshot of the oldest light in the cosmos. It was imprinted on the sky when the universe was just 380,000 years old. The image is based on data from the Planck mission. The data may contain information that points to energy bursts shortly after the Big Bang, so called hot spots.

© ESA/Planck Collaboration

The cosmic microwave background is a snapshot of the oldest light in the cosmos. It was imprinted on the sky when the universe was just 380,000 years old. The image is based on data from the Planck mission. The data may contain information that points to energy bursts shortly after the Big Bang, so called hot spots.

© ESA/Planck Collaboration

To the point

We are not yet able to observe the beginnings of the universe directly: this is prevented by a dense fog of gas and matter.
Signals from the very early universe: Scientists Leo Stodolsky and Joseph Silk from the Max Planck Institute for Physics propose ways in which researchers could nevertheless peer behind this curtain of cosmic background radiation to see the universe immediately after the Big Bang.
Violent bursts of energy and neutrinos play an important role in this.

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, behind which radiation and matter formed a hot plasma. Here, light and matter have been interacting constantly, thus preventing us from seeing into the time before.

Evolution of the universe: from ionized gas to transparent space. — Während der ersten drei Minuten nach dem Urknall entstanden die leichtesten Atome, Wasserstoff und Helium. Diese Atome waren anfänglich wegen der hohen Dichte und Temperatur ionisiert, ihre Elektronen bewegten sich frei. Während sich das Universum ausdehnte, kühlte das Gas ab und nach etwa 300.000 Jahren, als die Temperatur des Gases unter 5.000 Grad fiel, verbanden sich die Atomkerne wieder mit den Elektronen zu neutralen Atomen. Da noch keine Lichtquellen vorhanden waren, und weil selbst das Licht der ersten Sterne, die nach ein paar hundert Millionen Jahren entstanden, vom neutralen intergalaktischen Gas stark absorbiert wurde, wird diese Epoche des Universum das "Dunkle Zeitalter" genannt. Als sich dann mehr und mehr Sterne und leuchtende Schwarze Löcher bildeten, ionisierte deren Strahlung das umliegende Gas wieder, das dadurch nach und nach transparent wurde - der Nebel lichtete sich. Das "Zeitalter der Re-Ionisation" endete etwa eine Milliarde Jahre nach dem Urknall; das Universum entpuppte sich dann als ein Raum voller Licht und Leben.

© S.G. Djorgovski and the Caltech Digital Media Center

Während der ersten drei Minuten nach dem Urknall entstanden die leichtesten Atome, Wasserstoff und Helium. Diese Atome waren anfänglich wegen der hohen Dichte und Temperatur ionisiert, ihre Elektronen bewegten sich frei. Während sich das Universum ausdehnte, kühlte das Gas ab und nach etwa 300.000 Jahren, als die Temperatur des Gases unter 5.000 Grad fiel, verbanden sich die Atomkerne wieder mit den Elektronen zu neutralen Atomen. Da noch keine Lichtquellen vorhanden waren, und weil selbst das Licht der ersten Sterne, die nach ein paar hundert Millionen Jahren entstanden, vom neutralen intergalaktischen Gas stark absorbiert wurde, wird diese Epoche des Universum das "Dunkle Zeitalter" genannt. Als sich dann mehr und mehr Sterne und leuchtende Schwarze Löcher bildeten, ionisierte deren Strahlung das umliegende Gas wieder, das dadurch nach und nach transparent wurde - der Nebel lichtete sich. Das "Zeitalter der Re-Ionisation" endete etwa eine Milliarde Jahre nach dem Urknall; das Universum entpuppte sich dann als ein Raum voller Licht und Leben.

© S.G. Djorgovski and the Caltech Digital Media Center

The light that bears witness to the early universe at an age of 380.000 years is the cosmic microwave background radiation (CMB), which was discovered in 1964. This light was created when, about 380,000 years after the Big Bang, the universe had expanded and cooled to such an extent that the light was no longer coupled to the particles of matter: from then on, it has been free to travel through the entire universe. And as space has continued to expand ever since, the light waves of the cosmic background have also stretched and distorted – so much so that today, the background radiation can only be measured by radio observatories at wavelengths ranging from millimeters to centimeters.

Light is a carrier of information, and this faint radio light, which shines on Earth from all directions, transports the first direct information from the early universe directly to Earth. However, the authors suspect that there is a way to look 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 explosions could also emit neutrinos, weakly interacting particles that escaped from the clutches of hot matter earlier than light and could therefore witness an universe that is even younger.

Researchers Leo Stodolsky and Joseph Silk from the Max Planck Institute for Physics present three possible scenarios for observing an even younger universe than it is possible today. Neutrinos play a role in two of the three scenarios.

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. This radiation background would be very similar to the cosmic microwave background, but would consist of neutrino particles rather than light. The advantage of a neutrino background is that it would allow astronomers to look deeper, since neutrinos decoupled from the hot primordial soup earlier than light. 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 cosmic microwave background, in particular small regions with an out-of-equilibrium spectrum. The map of this radiation shows the light from the early universe as it arrives from all directions. Background radiation has been the subject of intensive research for decades, including the European Planck mission, in which the Max Planck Institute for Astrophysics played a key role. Plotting the light intensity across the various wavelengths contained in the radiation produces a light spectrum, which allowes the most accurate measurement of a black body. To explain: all objects with a certain temperature emit thermal radiation, such as a radiator in your home. The shape of the light spectrum reveals the temperature. This is also the case with the cosmic microwave background: the light indicates a temperature of the cosmos of only 2.7 Kelvin, i.e., 2.7 degrees Celsius above absolute zero of approximately -273 degrees Celsius. The measurements of the ice-cold universe are so accurate that the finest temperature fluctuations of hundreds of microkelvins (0.0001 degrees Celsius or Kelvin) around the average temperature of about 2.7 Kelvin can be mapped. This gives the map from above its patchy structure. Since these fluctuations are minimal, the map could also be drawn with a single color representing the mean temperature of the universe. In this map, Stodolsky and Silk hope to find fluctuations that cannot be explained by the known noise around the mean temperature.

With these three theoretical hypotheses, Leo Stodolsky and Joseph Silk provide the research community with concrete signatures that are worth searching for with telescopes and instruments.

MPP/MPG

How can we observe the birth of the universe?

To the point

Possible evidence of weak X-rays

A new low-energy neutrino background

Hot spots in the microwave background

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