Thanks to the discovery of the cosmic background radiation, scientists have reconstructed the variation in the temperature of the Universe up to today.
Our Universe, as we understand it, was not always as it appears today. The Universe, long ago, was more uniform, less lumpy, less evolved and, importantly, smaller. The last part is fundamental to our understanding of space, because it is an inevitable consequence of the fact that . As the Universe expands, which is counterintuitive to many of us, it also cools, as the photons within it not only dilute with increasing volume, but the wavelengths of individual photons also lengthen to longer lengths as the space through which they travel expands. That’s why we say that our cosmic past included not just a Big Bang, but a Big Bang heatin which not only the densities but also the temperatures were far higher than today. This conclusion leads to a simple yet interesting question: “so how the temperature of the Universe changes over time?” It’s a fascinating question, because although there is a relationship between time and temperature in our cosmic past, it’s not as simple as one might imagine.
The cosmic background radiation
One of the greatest discoveries of the entire 20th century was that of . In the 1940s, scientist George Gamow and his collaborators first worked out many of the consequences of a Universe emerging from a hot, dense, rapidly expanding state—something we now identify as the Big Bang. Gamow noticed that as the Universe expands, the wavelength of all forms of light within it are stretched by the expanding Universe, lengthening their wavelengths and lowering their energies. Consequently, he reasoned, if we extrapolated backwards, that light must be at shorter wavelengths, higher energies, and more tightly packed.
In other words, the Universe must have emerged not only from a denser state in the past, but also from a hotter one. Go back far enough, Gamow reasoned, one would arrive at a period in cosmic history when there were enough photons of sufficiently short wavelengths to make the formation of neutral atoms impossible: where as soon as a proton and an electron had found each other and temporarily created a neutral atom, they would have been blown up by one of those high-energy (ultraviolet) photons. This would correspond to a state in which the whole was nothing more than an ionized plasma: in which photons and electrons scattered profusely from each other and in which neutral atoms could not yet form stably due to the extremely high temperatures.
However, as the Universe expanded and cooled, the wavelengths of those critical photons would have lengthened, eventually lowering their energies below the ionization threshold of neutral atoms. Eventually, after enough time had passed, the Universe would fill with neutral atoms and those remaining photons, now of a wavelength too long to ionize the atoms it was encountering, would simply “free-flow,” that is, travel in a straight line without being absorbed. As the Universe continued to expand, leading to ever-longer wavelengths and ever-lower temperatures. Ultimately, today, billions of years later, that remaining radiation bath would have been only a few degrees above absolute zero.
Although it had been predicted by Gamow as early as 1945, and subsequent details were worked out by Ralph Alpher and Robert Herman in 1948, research of this background did not receive much interest until the 1960s. Bob Dicke and his team at Princeton, including legendary scientists Jim Peebles, Peter Roll, and David Wilkinson, sought to design and fly a radiometer that would search for the signature (now microwave wavelength) of this afterglow: now known as the cosmic microwave background but then known, poetically, as the primordial fireball. However, before they could do so, a serendipitous discovery by Arno Penzias and Robert Wilson revealed the presence of this afterglow of low-energy radiation in all directions of the sky. The cosmic microwave background.
Carbon monoxide
This was the first detection of what is now considered the unmistakable clue that . Over the years, further discoveries were added which led to the conclusion that this radiation was actually:
- isotropic, i.e. the same in all directions,
- a black body in nature, exactly as predicted by the Big Bang,
- uniform in temperature at approximately 1 part in 30,000,
which even today remain properties of this radiation that no alternative framework can coherently explain. When you add observations of the expanding Universe, the relative abundance of light elements, and the observed evolution of galaxies and the formation of cosmic structure over time, the Big Bang reigns supreme as our best (and currently only valid) ) theory of the early Universe.
This all sounds very theoretical, of course, and so one might wonder if there is anyone to confirm it. We precisely measured the cosmic microwave background and found that its properties today are that the Universe has a temperature that corresponds to 2.7255 K and where the temperature fluctuations, or deviations from that mean value, are only tens or hundreds of microkelvins. But the existence of that radiation today does not necessarily imply that it existed, as expected, at higher temperatures in the past and at temperatures that correspond to the physical extent of the expanding Universe relative to its extent today.
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Fortunately, there is a way to verify this statement too. One of the most ubiquitous molecules in the Universe is carbon monoxide: a simple molecule of CO. Because of carbon monoxide’s propensity to have its spin levels excited at relatively low energies, at energies that correspond to photons of microwave frequencies, it can be used as a cosmic thermometer, where the absorption characteristics of carbon monoxide can appear in the spectrum of a distant.
As first reported in a 2011 study, the carbon monoxide signature shows, with only very small uncertainties, that the temperature of the cosmic microwave background in the distant past was indeed hotter and correlated with the size of the Universe, exactly as expected. To date we therefore know that the Universe is composed of approximately 5% normal matter, 27% and 68% dark energy, with a small amount of neutrinos (~0.1%) and an even smaller fraction of photons (~0.01%) added. We also know that the Universe is expanding right now, and despite controversy over a problem known as the Hubble strain, we know with very high precision how fast the Universe is actually expanding today.
In its own small way, Passione Astronomia helps you understand how the universe works. And the universe works better if the people who are part of it are well informed: if they have read nonsense, lies, poisons, then it ends up as it ends up. It’s not going very well right now. This is why it is important that someone explains things well. Passion Astronomy does its best. !
The evolution of the temperature of the Universe
Based solely on this information, we can calculate backwards, from the present, to any time in the Universe’s past, and learn what the relationship is between temperature and time. It turns out that there are three separate periods in cosmic history that gradually merge into each other, where the relationship between temperature and time is distinct, following different rules in different periods, depending on which form of energy is dominant and mainly responsible for.
At first, radiation, driven by photons and neutrinos, is the dominant factor in the Universe. This period persists from the beginning of the hot Big Bang for the first few seconds, minutes, hours, days, years, decades, centuries and millennia. Then, around 9,000 years onwards, matter begins to take over, becoming the dominant form for millions and even billions of years, all while the Universe continues to expand and wavelengths continue to lengthen. Finally, about 6 billion years ago, the density of matter became so diluted that matter began to become important and now dominates the expansion of the Universe. As the Universe expands, that is, as the distance between two points increases, the temperature of the Universe decreases proportionally.
This means that the temperature of our Universe today, at 2.7255 K, was higher in the past. Some fun facts are that when the Universe was an age of:
- 13.8 billion years, T = 2,7255 K,
- 10 billion years, T = 3,62 K,
- 7 billion years, T = 4,77 K,
- 1 billion years, T = 18,12 K,
- 100 million years, T = 83,4 K,
- 10 million years, T = 379 K,
- 1 million years, T = 1633 K,
- 25,000 years, T = 13.700 K,
- 1000 years, T = 73.600 K,
- 1 anno, T = 2.370.000 K,
and so on, where for every factor of 100 more than in the past, the temperature becomes ten times higher.
This is the relationship: the Universe changes temperature over time, and the temperature of the Universe always drops. Interestingly, you can still have regions of space, such as the interior of , where the rapid expansion of matter can create a colder location than the deepest depths of intergalactic space. These temperatures are reached similarly to the low temperatures to which the cosmic microwave background radiation eventually moves: through rapid and relentless expansion. In the case of the Boomerang Nebula, it’s due to the expansion of matter, not the expansion of space, but the same principle still applies.
We understand our cosmic history well enough that we can now trace back to any time in the Universe’s past and know, with only extremely small uncertainties, what the temperature of the Universe was in those early times. We can probe this over a huge range of conditions, from when the Universe was just a tiny fraction of a second to when it was many billions of years old, as well as every stage in between.
When you ask about the temperature-time relationship, you are asking about the physics that governs the Universe at all times. 100 years ago, the answers to these questions would have been purely theoretical; today, we have an overwhelming body of evidence that supports this picture. This is quite an achievement for , and something that everyone, not just physicists and astrophysicists around the world, can appreciate about our understanding of the Universe.
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