The reason why the universe has not cooled enough for liquid hydrogen or liquid helium to exist in space is primarily due to the expansion and dynamics of the universe, as well as the effects of other astrophysical processes. Let's explore these factors:
Expansion of the Universe: The universe has been expanding since the Big Bang. As the universe expands, the average energy density of photons decreases, leading to a cooling effect. However, the expansion also causes a decrease in matter density. The combination of these effects means that the universe has not cooled enough for the formation of liquid hydrogen or helium under normal conditions.
Cosmic Microwave Background Radiation (CMB): The afterglow of the Big Bang, known as the cosmic microwave background radiation, is one of the key pieces of evidence for the Big Bang theory. The CMB represents the residual heat from the early hot and dense phase of the universe. While the CMB has cooled significantly since its formation, it still corresponds to a temperature of around 2.7 Kelvin (-270.45 degrees Celsius or -454.81 degrees Fahrenheit) today. This temperature is too high for liquid hydrogen or helium to form.
Cosmic Structure Formation: As the universe expanded and cooled, gravity played a crucial role in the formation of structures like galaxies, stars, and planets. These structures are formed through the collapse and condensation of gas clouds, leading to localized regions of higher density. Within these structures, temperatures and pressures can be much higher than the average temperature of the universe, preventing the formation of liquid hydrogen or helium in most regions of space.
Interstellar Medium: The space between stars, known as the interstellar medium, contains gas and dust. This medium plays a role in shaping the physical conditions within galaxies. The interstellar medium is not uniformly cool and can have various temperatures depending on the region. While there are colder regions within interstellar space, the overall temperature is not low enough for liquid hydrogen or helium to exist under typical conditions.
In summary, the cooling of the universe and the formation of liquid hydrogen or helium are influenced by complex factors such as the expansion of the universe, the presence of the cosmic microwave background radiation, the dynamics of cosmic structure formation, and the properties of the interstellar medium. These factors collectively contribute to the absence of liquid hydrogen or helium in most regions of space. However, there are extreme environments such as very cold interstellar clouds where these elements can condense and form liquids.