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Why the Universe Cooled as It Expanded: A Cosmic Paradox Contradicting Earthly Physics

PHYSICXION: The difference between temperature-volume relationships in everyday scenarios and the expansion of the universe after big bang.

 

cover image of Why the Universe Cooled as It Expanded: A Cosmic Paradox Contradicting Earthly Physics on physicxion


Why the Universe Cooled as It Expanded: A Cosmic Paradox Contradicting Earthly Physics


In the beginning, there was nothing. No stars, no galaxies, no planets—just a singular point containing all the energy, matter, and forces that would one day create everything we see in the universe. This was the moment of the Big Bang, an unimaginable burst of creation that marked the birth of time and space itself.

The magical First fraction of the second of birth

The first second after the Big Bang (approximately 13.8 billion years ago) was nothing short of magical, though "magical" in this case meant searingly hot and chaotic. The temperature was unfathomably high—around 10 billion Kelvin—so intense that the fundamental particles of matter, like quarks and gluons, could not yet come together to form atoms. The universe was a scorching, dense soup of raw energy, where particles collided with such force that they could not hold together for long. Even light struggled to escape this early inferno, constantly absorbed and re-emitted by the particle soup. In this blinding furnace, the fundamental forces of nature—gravity, electromagnetism, and the strong and weak nuclear forces—were in a state of flux, still shaping themselves into the forms we know today.

In this cosmic cauldron, time was racing forward, and in less than a second, the universe expanded at an exponential rate, a phase known as inflation. This rapid expansion caused the fabric of space to stretch, pulling everything farther apart. Even as the universe grew larger, the heat and energy from the Big Bang remained. Yet, something incredible began to happen: as space expanded, the temperature began to fall.

This cooling was a direct result of the universe's adiabatic expansion—the same principle that causes the air in a balloon to cool when it expands. The energy, once tightly packed into a minuscule point, was now spread over an ever-increasing volume. The particles, once jostling together at incomprehensible speeds, began to slow down as the universe grew larger and more spacious.

Within the first three minutes, a remarkable transition took place. The temperature had cooled enough—dropping to about one billion degrees—that protons and neutrons could finally begin to form the first atomic nuclei. The elements that would one day make up stars and galaxies, and even us, had started to take shape. This period is often referred to as nucleosynthesis, the moment when the universe produced the first nuclei of hydrogen, helium, and trace amounts of other light elements.

For the next 380,000 years, the universe continued to cool and expand, but it was still too hot for atoms to form. Electrons were freely roaming, unable to attach themselves to nuclei because of the high energy around them. The universe remained a glowing, opaque plasma. But as it expanded, the temperature continued to fall—reaching about 3000 K. Finally, a breakthrough: the temperature dropped enough for electrons to bond with protons, creating the first stable atoms. This allowed light to travel freely for the first time, filling the universe with a warm glow—the Cosmic Microwave Background (CMB).

As billions of years passed, the universe continued its silent expansion, cooling as it went, until it became the vast, starry cosmos we know today. And yet, in the faint whispers of the CMB, we can still hear the echoes of that first magical moment of creation.


The Adiabatic Cooling

As the universe expands, its temperature decreases due to a process called adiabatic cooling. Here's a breakdown of why this happens:

1. The Big Bang and Initial High Temperature:

  • Right after the Big Bang, the universe was extremely hot and dense. The temperatures were so high (↝ 10^10 K) that matter couldn't even form; everything existed as a hot plasma of particles like quarks and photons.

2. Expansion and Reducing Energy Density:

  • Over time, the universe has been expanding. As it expands, the energy density (the amount of energy per unit volume) decreases because the same amount of energy is now spread over a larger volume.

3. Photon Redshift and Energy Loss:

  • Light and other forms of radiation, including the Cosmic Microwave Background (CMB), were emitted early in the universe's history. As the universe expands, the wavelength of these photons stretches (this is known as redshift). Since energy is inversely proportional to wavelength, as the wavelength increases, the energy of the photons decreases. Lower-energy photons correspond to lower temperatures.
illustration for expansion of universe from big bang resulting Energy loss with photon redshift resulting cooling on physicxion
Illustration of photon redshift with the expansion of the universe with time

4. Adiabatic Expansion:

  • As the universe expands, it does so without significant heat exchange with the surroundings (because there are no surroundings in a cosmological sense). This type of expansion, where the system expands and cools without gaining or losing heat, is known as adiabatic expansion. In this case, the gas (initially a hot, dense plasma) cools as it expands because its internal energy decreases.

5. Cosmic Microwave Background (CMB) Evidence:

  • The CMB, which is the leftover radiation from the early universe, provides direct evidence of this cooling. Initially, when the universe was only about 380,000 years old, the temperature was about 3000 K. As the universe expanded over billions of years, this radiation has cooled, and today the CMB is observed at a much lower temperature of about 2.7 K (just above absolute zero).

6. Connection Between Temperature and Volume:

  • In physics, the temperature of a gas is related to the average kinetic energy of its particles. As the universe expands, the distance between particles increases, and their average kinetic energy (and thus temperature) decreases because there’s less energy available per particle.


With the increase in temperature volume increases but why for the universe it decreased? differentiating these two phenomena

Coming to a great and thought-provoking question! The difference between temperature-volume relationships in everyday scenarios (like gas in a balloon) and the expansion of the universe after the Big Bang comes down to the nature of the systems we’re discussing, and how energy behaves within them. Let me walk you through how to differentiate these two phenomena:

1. Everyday Phenomenon: Temperature-Volume Relationship

In our everyday experience, we usually encounter gases trapped in a container or under the influence of pressure, like air inside a balloon. According to the ideal gas law (PV = nRT), for a given amount of gas, if the temperature increases, its volume expands (assuming pressure is constant). This happens because the molecules in the gas gain energy and move faster, pushing against the walls of the container, and causing it to expand.

Key points in everyday temperature-volume scenarios:
  • The system (e.g., the gas in a balloon) is in a fixed space or under a fixed pressure.
  • When you add energy (heat), the molecules move faster, increasing the temperature and volume simultaneously.
  • It's a localized process, where energy is transferred within a specific space or container.

2. Expansion of the Universe: A Different Process

Now, when we talk about the expansion of the universe after the Big Bang, we are dealing with something vastly different—a cosmological process that doesn't fit the ideal gas law in the same way.

After the Big Bang, the universe itself was expanding. Space-time itself stretched. This was an adiabatic process (meaning there wasn’t heat exchange like in our gas example). As the universe expanded, the energy that was originally concentrated in a small volume was spread over a much larger area, causing the energy density (energy per unit volume) to decrease. Because temperature is closely tied to the average energy of particles, this spreading out of energy caused the temperature to drop.

Key Differences in the Universe's Expansion:

  • Space itself is expanding: In the universe’s case, it’s not just a balloon expanding in space—space-time itself is stretching. This stretching pulls galaxies and particles farther apart, and as they spread out, the energy density drops.
  • Adiabatic expansion: Unlike heating a gas, no external heat is added. Instead, as the universe expands, it naturally cools because energy is distributed across an increasing volume of space.
  • Cosmic cooling: The expansion of the universe caused cooling because energy was "diluted" over a larger volume. Right after the Big Bang, the universe was a hot, dense plasma, but as it expanded, the temperature decreased drastically.

Why They Seem Opposite:

In a contained system, like a balloon, when you heat something, you increase the temperature, which increases the volume. However, in the case of the universe, the opposite seems to occur. As the universe expands, energy spreads out over a larger volume, leading to cooling, rather than heating.

In short, the everyday temperature-volume relationship is about adding energy to a confined space and watching it expand, while the universe’s expansion is about the stretching of space-time, causing the existing energy to be spread out and the temperature to drop as a result.

An Analogy:

Imagine you have a cup of hot coffee. If you pour that coffee into a much larger container (say, a swimming pool), the heat from the coffee will be spread out across a much larger volume of water. As a result, the temperature of the water-coffee mix will be much cooler, even though no heat has been "removed." Similarly, the universe expanded, and the energy within it was spread out, causing the temperature to drop.

This difference highlights the unique nature of cosmological expansion, where the fabric of the universe itself stretches, leading to cooling, rather than the addition of heat causing expansion like we see in more localized, everyday situations.

Is it a topological expansion of space-time fabric?

The expansion of space-time after the Big Bang is indeed something we can think of in terms of topological expansion, but it’s quite different from everyday experiences like blowing up a balloon. While the balloon analogy helps us initially imagine the stretching of space, it has its limitations because it suggests expansion into something (the air around the balloon). In reality, space-time doesn’t expand into anything—it’s more of an intrinsic expansion where the fabric of space itself stretches.

Let’s explore how one might imagine this without relying solely on the balloon analogy:

1. Imagine Stretching Space Itself

In our everyday experience, we usually think of objects in space, like galaxies, planets, or stars, existing within space. But with the expansion of the universe, it’s space itself—the very framework that holds everything together—that is expanding. This means that every point in space is moving away from every other point (on large scales), not because objects are moving through space, but because the space between them is increasing.

You could imagine this by thinking about two dots on a rubber sheet. If you pull on the edges of the sheet, the space between the dots grows, even though the dots themselves aren’t moving on the sheet. In this way, space between galaxies is expanding.

2. The Infinite Grid Analogy

Another way to picture it is to think of the universe as an infinite grid. Imagine that space is represented by the intersections of the grid lines. As the universe expands, the grid itself is stretching—each square in the grid is getting bigger. But crucially, the grid isn’t expanding into anything; it’s just stretching internally.
  • No edges: Unlike a balloon or a physical object, the universe has no edges in this analogy. So the grid can keep expanding infinitely without ever encountering a "boundary."
  • Everywhere expands: The key here is that expansion is happening everywhere at once, not just at the "edges" like a balloon. Galaxies are not moving through space—they are being carried apart because the "fabric" of space itself is stretching.

3. Expanding Raisin Bread Model

This is another helpful analogy. Imagine a loaf of raisin bread baking in the oven. The raisins represent galaxies, and as the dough rises, the distance between the raisins increases. Just like the bread dough is expanding in all directions, the space-time fabric between galaxies is expanding, causing them to move apart. The raisins (galaxies) are not moving through the dough—they are simply being carried along as the dough (space) expands.

However, like the balloon, this analogy also has limitations. The loaf of bread is still expanding in a larger space (the oven), while the universe’s expansion is happening without there being anything external into which it’s expanding.

4. Topological Expansion in Curved Space-Time

The mathematical view of space-time expansion can be described using topology and geometry. In the framework of General Relativity, space-time is a curved manifold that can stretch and deform. When we talk about the expansion of the universe, we’re referring to the changing geometry of this manifold.

In this context:
  • No external space: The expansion doesn’t mean the universe is expanding into a higher-dimensional space like a balloon expands into the air. Instead, the metric (a mathematical object that defines distances in space-time) is changing. This is what we mean when we say the universe is expanding.
  • Curved space-time: Space-time can be flat or curved depending on the distribution of matter and energy in the universe. The Big Bang model assumes a space-time that’s expanding and possibly curved, where distances between objects are increasing.

5. Time as Part of Expansion

  • Another crucial aspect to consider is that it’s not just space that’s expanding—it’s space-time. This means that time is inherently tied to this expansion. When we say the universe is expanding, the way we experience time and space is dynamically evolving. The rate at which galaxies are moving apart is tied to how we experience time itself on cosmological scales.

6. More Than 3 Dimensions?

Some theories in physics (like string theory) suggest there could be more than three spatial dimensions, and some of these dimensions might be tightly curled up or "compactified." While we don't have direct evidence of this extra-dimensional space, it’s a reminder that our everyday, three-dimensional experience of the world might not be the full picture.

In terms of imagining this, consider that space itself could be "curved" in ways that are hard to visualize. Expansion might occur in higher-dimensional spaces, where the three-dimensional space we perceive is just part of the story.

Key Takeaways:

  • The expansion of space is not about objects moving through space like we see in our daily experiences. Instead, it’s space-time itself stretching.
  • No edge or boundary: The universe isn’t expanding into a "larger" space but is expanding internally, with distances between galaxies increasing over time.
  • Intrinsically different: While a balloon or raisin bread model helps us get an initial sense, the universe’s expansion is far more profound and governed by the geometry of space-time itself.

Final Thought:

It's hard for us to fully visualize the expansion of space-time because we’re confined to living within it and experiencing the universe in three spatial dimensions. The beauty of the universe’s expansion is that it challenges our intuition and pushes the limits of human imagination, offering an incredible insight into the deep and complex nature of the cosmos.


Role of Dark Energy 

Dark energy plays a pivotal role in the expansion and cooling of the universe. It is a mysterious and poorly understood form of energy that permeates all of space and is responsible for driving the accelerated expansion of the universe. As the universe continues to expand, the influence of dark energy becomes increasingly dominant.

Here’s how dark energy is connected to the expansion and cooling of the universe:

1. Acceleration of the Universe's Expansion:

In the early universe, the expansion was dominated by matter (both normal and dark matter), and gravitational attraction was the primary force. However, around 5 billion years ago, the repulsive force of dark energy overtook gravity, causing the universe to expand at an accelerated rate. This acceleration stretches space more rapidly, which in turn causes radiation (photons) to lose energy through redshifting. As a result, the overall temperature of the universe continues to decrease.

2. Cosmic Redshift and Cooling:

As space expands, the wavelengths of light and other forms of radiation also stretch out, causing the radiation to redshift to lower energies. This leads to a decrease in the temperature of the cosmic microwave background radiation (CMB), the relic radiation from the Big Bang. Dark energy, by accelerating the expansion of the universe, enhances this cooling effect, ensuring that the temperature of the universe continues to drop over time.

3. Dilution of Matter:

As the universe expands, the density of matter (both normal matter and dark matter) becomes increasingly diluted. With dark energy driving the expansion faster and faster, the universe’s energy is spread more thinly over an increasing volume of space. This dilution also contributes to the cooling, as the energy density decreases with the growing size of the universe.

4. Future of the Universe:

If dark energy continues to dominate and the expansion of the universe accelerates unchecked, it is theorized that the universe could eventually experience a "Big Freeze" or "Heat Death." In this scenario, the universe expands so much that all matter, stars, and galaxies are spread far apart, and the temperature of the universe asymptotically approaches absolute zero, leading to a cold and dark universe.


Conclusion:

In short, the temperature of the universe decreases with expansion because the universe's energy is spread over a larger volume, and radiation loses energy (redshifts) as space itself stretches. This cooling process is a fundamental aspect of the universe’s evolution since the Big Bang.