The radio telescope in Paraíba and the mysteries in the Universe – 10/06/2023 – Science

Before stars and planets, black holes and white dwarfs appeared, and even before the first atoms and rays of light, the Universe was already reverberating with something surprising — sound.

The primordial hum of the Universe traveled at more than half the speed of light, cutting through the superheated plasma of photons, baryons and dark matter. It arose from a tug of war between powerful fundamental forces, which generated sound waves in that electrically charged soup of particles.

When the Universe was “only” a few hundred thousand years old, the plasma disappeared like morning fog. And the Universe quickly fell into profound silence.

But it is still possible to capture echoes of these first sound waves that propagated throughout the early Universe, if we know where to look.

The oscillations created by these waves in the plasma left a permanent mark on the distribution of matter throughout the Universe. And these wobbles also give astronomers clues into one of the deepest mysteries of our current Universe: that mysterious force known as dark energy.

The primordial sound waves — also known as baryon acoustic oscillations (BAOs) — were formed when particles in the early Universe began to come together, attracted by gravity.

“The gravitational force of dark matter in the early days of the Universe created ‘wells of potential’, which attracted plasma into its interior”, according to Brazilian physicist Larissa Santos, professor at the Center for Gravitation and Cosmology at Yangzhou University, in China.

But the plasma was so hot that it also created another force, in the opposite direction. “The photons created radiation pressure that fought against gravity and pushed everything back to the outside. This fight created acoustic oscillations – sound waves”, explains the professor.

BAOs erupted from countless wells of potential, forming concentric spheres of expanding sonic energy. They crisscrossed each other, sculpting the plasma into complex and dazzling three-dimensional interference patterns.

If there were humans living in the time of “baryon acoustic oscillations” (BAOs), they would not have heard any noise. The sounds were about 47 octaves below the first piano note. Their wavelengths were gigantic – about 450 thousand light years.

These inaudible and incredibly deep rumbles traveled through a medium incapable of being penetrated by even our most powerful telescopes.

In search of ‘fossil records’

The more deeply we look at the Universe, the more we return to its history. This is due to the time it takes for light to reach us.

But we can only see so far because the electrical charges of the protons and electrons released in those early stages of the Universe’s life scattered and diffused the light, creating an impenetrable random glow.

Meanwhile, BAOs created patterns in the medium that swung outward. Therefore, we can observe its evidence in the current Universe.

The European Space Agency’s Planck Space Telescope managed to capture echoes of BAOs from the early Universe, which scientists translated into audible frequencies.

Tinnitus is made up of a low tone with higher overtones. It has been processed to produce a sound file with intense noises that can be heard by humans.

When it reached around 379,000 years old, the Universe cooled enough for protons and electrons to pair up, forming the first neutral hydrogen atoms. The plasma then disappeared, which made the Universe suddenly transparent and allowed the transmission of light.

At the same time, the battle between radiation and gravitation came to an end. The BAOs ceased and the Universe fell silent.

A jet of light energy then began to spread throughout the Universe. It was so powerful that it still resonates through radio telescopes today, attracting physicists as a signal of the cosmic microwave background radiation (CMB), 13 billion years later.

The CMB is the oldest and most detailed visual record of the early Universe. It offers scientists a “fossil record” of the first sounds of the cosmos.

“We see [os sons] imprinted in the cosmic microwave background radiation and also in the structure of the Universe on a large scale”, according to Santos. The Brazilian physicist is participating in a new international radio telescope project to analyze the modern echoes of that ancient song.

“Its signature is found in the slightly excessive number of pairs of galaxies that are separated on a fixed scale of 150 Megaparsecs — about 500 million light years,” explains the professor.

Project under construction in Paraíba

BAO’s signatures are not just indications of what the Universe’s first sounds would be like. They also serve as a standard for measuring the effects of another invisible phenomenon: dark energy.

Dark energy makes the Universe expand. Its effects are everywhere, but its nature is unknown.

Studying the scale of BAO signatures at different distances from Earth tells how the effects of dark energy have altered the history of the Universe.

“We call it a standard ruler,” says Santos. “We have this fixed scale. From its apparent variations, we can know how the Universe has evolved over time.”

Larissa Santos is part of the international project responsible for the Bingo radio telescope, currently under construction in Paraíba. Bingo is the English acronym for “Baos of Integrated Neutral Gas Observations”.

The radio telescope will be tuned to the characteristic radiation signatures of hydrogen — the simplest, oldest and most abundant atom in the Universe.

Hydrogen atoms release radiation with a wavelength of 21 centimeters. This length is invisible to the human eye, but can be detected by radio telescope.

Dark energy “stretches” radiation from more distant hydrogen clouds. As a result, the wavelength observed here on Earth increases. The greater the distance, the longer the wavelength.

“You choose the frequency of the radio telescope according to the period of the Universe you want to measure”, explains Santos.

The Bingo radio telescope was designed to map the distribution of hydrogen between one billion and four billion light years ago – which is relatively close, on the cosmic scale of time and space.

Bingo’s two enormous parabolic mirrors reflect this primordial radiation onto a set of 50 directed wave detectors, known as “horns.”

The telescope’s mobile base is the planet where it is being built. The Earth’s rotation moves the equipment under the stars, sweeping an area of ​​the sky measuring 15 by 200 degrees.

Using complex statistical calculations, Professor Larissa Santos will analyze the data to locate millions of galaxies by examining the relative distances between them. With this, it will be possible to study in more depth how dark energy affected the patterns of BAOs at that time.

“Bingo will look at the later Universe, after dark energy dominated the expansion. It’s a great complement to other experiments,” she said. And many of these other experiments have already begun or are planned.

‘Greedy’ approach

“Mapping hydrogen intensity, in principle, can measure anything in the Universe between the present day and the CMB. It’s an immense volume to be explored,” says physics professor Cynthia Chiang, who studies hydrogen density at the University McGill in Montreal, Canada.

“Bingo and other similar experiments look for gases that are inside galaxies. They are a marker of where the matter is”, explains the professor.

Instruments tuned to relatively nearby regions are of interest to Chiang, but she also wants answers about the rest of cosmic history.

“My approach is very greedy,” says Chiang, laughing. “I am organizing an experiment tuned to frequencies corresponding to the ‘Dark Ages’.”

“This is the period immediately following the formation of the microwave background. We never had access to the cosmology of that period because it is very, very difficult”, according to the professor.

Between the “surface of the last scattering” (when the baryonic plasma gave way to the CMB) and the “cosmic dawn” (when the light of the first star shone), there is a gap of 250 to 350 million years. The BAOs left clouds of hydrogen grouped in thin streaks, like receding sea waves that leave ripples in the sand.

Before Chiang can access the 21-cm radiation from that time, she needs to design experiments to exclude more recent signals from our own galaxy, which could mask older data.

“This first experiment will not yet reach cosmology,” she explains. “The goal is to map the Milky Way’s emissions at these frequencies at very high resolution, so we can know what the sky looks like on the first pass. Then, we hope to be able to subtract that and get to the cosmology.”

“As the name suggests, in the Dark Ages, the Universe was a very dark and monotonous place”, continues the professor. “There, the signal you receive is a nearly uniform 21 cm emission from that hydrogen wall.”

“But there are subtle fluctuations in brightness that correspond to higher and lower densities. You get tiny hot and cold spots.”

For the professor, the CMB is like a still photograph that captures, in impressive detail, a fundamental moment in cosmic evolution. But mapping hydrogen density in the Dark Ages would also capture hundreds of millions of years immediately afterward.

“You can probe a three-dimensional volume,” explains Chiang. “If you can measure the same kind of information as the CMB, but reflected from hydrogen, you get a lot more data and potentially can constrain the cosmological parameters even further.”

“If we get there, it will be wonderful. But it’s a very, very long road.”

Cosmic inflation

The experiments planned by Cynthia Chiang and the Bingo telescope add to a growing suite of innovative observing instruments that aim to unravel the history of BAOs, the large-scale structure of the Universe, and the invisible dark energy that separates galaxies.

“When we measure the sky, we measure everything”, explains Larissa Santos. “The CMB, neutral hydrogen, the sources of galaxies, all these kinds of things. We need to be able to recognize what is a cosmological signal and what is something else.”

Santos also hopes that BAOs will reveal even more about the Universe’s past, drilling into the 379,000-year-thick wall of plasma to provide data on the previous split second — the Universe’s “inflationary era.” After all, most cosmologists believe that, in that era, space expanded at a speed greater than that of light.

Cosmic inflation is a widely accepted theory about the evolution of the Universe from its original tiny, hot, dense state to the cosmos we see today.

This theory has gone through many models, variations and simulations. It offers many consistent predictions that have been tested and verified, although there is no direct evidence for this.

“Many inflationary theories have already been discarded by our observations”, according to Santos. “With the measurements we want to see, we can determine which theories best fit the measurements before moving forward.”

Baryon acoustic oscillations have only existed for a few hundred thousand years. But they helped create the story of the invisible Universe from beginning to end. Now, they help scientists tell this story.

Read the original version of this report (in English) on the BBC Future website.