Why is space so cold ?

© Geralt/Pixabay

The image of a starry sky often evokes warmth and light. Yet our Universe is an extremely cold place, with an average temperature of around -270° C. How to explain this apparent paradox ? Why is the universe, populated by billions of incandescent stars, so cold ?

To understand this phenomenon, we must first reconsider our concept of temperature. Far from being an intrinsic property of space, it is in fact a measure of the agitation of the particles that compose it. This seemingly simple definition opens the door to a fascinating exploration of the thermal extremes of our cosmos.

The vacuum of space: a thermal desert

In reality, temperature is a measure of the average kinetic energy of particles. Imagine a jar of jam. Inside it, tiny particles (the “jam molecules” /em> ») are constantly moving. When you heat this pot, these molecules start to move faster and faster, bouncing off each other with boundless energy. It is this agitation that makes the jam hot. Temperature is therefore a form of indicator of this agitation: the faster the particles move, the higher the temperature.

Conversely, when it is cold, the particles slow down, they move less and stay closer together. This is why an ice cube, for example, is cold: its particles are not very agitated. Emily Hardegree-Ullman, a professor of astronomy at Colorado State University, sums it up simply: “Space is mostly cold because it's almost empty. No matter, no heat ». An observation that may seem counterintuitive when we know that billions of celestial bodies populate space: black holes, gigantic planets, stars or asteroids.

In the quasi-absolute vacuum of interstellar space, particles are so rare and so far apart that they cannot transfer heat efficiently. Conduction and convection, the main heat transfer mechanisms on Earth, are practically non-existent. Only radiation, which is less efficient, allows some propagation of thermal energy.

Islands of heat in an ocean of cold

Despite this general coldness, the Universe is not uniformly glacial. Our solar system is a perfect example: from the Mercury furnace oscillating between 427°C during the day and -173°C at night, to the solar corona reaching 2 million degrees. Further away, the quasar 3C273, a celestial object present in the constellation Virgo, beats all records with an estimated temperature of 18 trillion degrees, or 3.27 billion times that of the surface of the Sun.

200% Deposit Bonus up to €3,000 180% First Deposit Bonus up to $20,000

However, thesehot spots remain exceptions in the cold immensity of the universe. The average temperature of space, determined by the cosmic microwave background – a thermal remnant of the Big Bang – is only 3 kelvins, or about -270°C. This temperature continues to drop over time, as the universe expands.

At the limits of absolute zero

The quest for absolute cold paradoxically brings us back to Earth. If the Boomerang Nebula holds the cosmic record for coldness with a temperature of 1 kelvin, it is in our laboratories that the lowest temperatures have been reached. In 2021, German researchers succeeded in cooling rubidium atoms to 38 trillionths of a degree above absolute zero.

This race towards absolute zero (-273.15° C) However, it runs up against the laws of quantum physics. As Hardegree-Ullman explains: “For a system to reach absolute zero, all of its particles would have to be at complete rest » However, the Heisenberg uncertainty principle teaches us that it is impossible to know simultaneously and precisely the position and speed of a particle.

Although space is extremely cold, the lowest temperatures ever observed have been artificially created on Earth, under laboratory conditions controlled from A to Z. Nevertheless, it remains our best teacher in the understanding of extreme thermal phenomena. By studying these vast icy expanses, we continue to push the boundaries of our knowledge about the very nature of heat and cold. So the next time you look up at the twinkling stars, remember that their light passes through an ocean of near-absolute cold before reaching your eyes. A true cosmic paradox.

• Space is generally cold because it is nearly empty, with few particles to carry thermal energy.
• Despite this general coldness, there are extreme hot spots like stars and quasars.
• The lowest temperatures in the universe have been artificially created on Earth, even surpassing the natural cold of space.

📍 To not miss any Presse-citron news, follow us on Google News and WhatsApp.

[ ]