Knowing when the first stars were formed, soon after the Big Bang, and understanding how they produced the building blocks of the first galaxies is an important scientific question and one of the primary science goals of JWST. We know that the elements that are needed for life and modern technology, such as carbon, silicon and gold, were ultimately created in early stars—but we don't currently have a good understanding of how this happened. From the article "James Webb telescope: How it could uncover some of the universe's best-kept secrets," by Martin Barstow.
Before
moving on to the different kinds of postulates that can be used to quantify and justify quantum
mechanics, I'll take a brief detour to an unexpected and rather spectacular
example of a simple thermodynamic system: the universe
in its early stages of existence, just before it cooled enough (to about 3000 K)
for atoms to be formed. This was 380,000 years after the Big Bang, when
the signature of the cosmic microwave background radiation (CMB) we observe today
was left behind.
As
I mentioned previously, our main man Harry Robertson defines a simple
system as a “bounded region of space that is macroscopically homogeneous,”
and says of the possible boundaries for such a system that they can be material
boundaries or “described by a set of mathematical surfaces in space.”
Mark
Whittle, an astronomy professor at the University of Virginia, in Lecture 16 of
“Cosmology: The History and Nature of
Our Universe,” gives
the conditions that make the early universe simple:
1.
The young universe is almost perfectly homogeneous (“it’s a smooth gas”).
2.
It contains (relatively) simple components: light, electrons, nuclei, and dark
matter. “And furthermore, the light, electrons, and nuclei are all tied tightly
together into a single coherent photon-baryon gas.” So the early universe was a
photon-baryon gas plus dark matter.
3.
The gas of nuclei, electrons and light is in thermal equilibrium.
4.
The deviation from exact homogeneity—the lumpiness—is very slight, so it’s
considered to be in the “linear regime,” and the physics of how the lumpiness
grows in time is simple (linear, not nonlinear).
The
only other condition we need in order for the early universe to fit the criteria for a simple system is a boundary—and this is more of a problem. In Lecture 15, “Primordial Sound—Big Bang
Acoustics,” Mark Whittle says, “Although the universe has no spatial
boundary, it is bounded in time. At a given time, e.g., the [time of formation
of the] CMB, regions of a specific size are caught at maximum or minimum
compression or rarefaction, and these specific region sizes create the
strongest patches on the CMB, giving the harmonics … .”
The slight deviations in the density of the almost-homogeneous gas of electromagnetic radiation, simple nuclei (protons+neutrons), and electrons result in acoustic waves in the gas that bounce in and out of the denser regions. These waves each have a fundamental frequency (about 50 times lower than the low end of human hearing range) and harmonics. And what good are harmonics? Different harmonics make different musical instruments playing the same note (the fundamental) sound different. The size and shape of an object and the components that make up the object can be roughly determined by what harmonics it produces.
Studying
the harmonics may not be able to give a unique or exact size, shape, and
material composition of a vibrating object, but given a limited set of possible
components in the material and other information, the harmonics give the best
possible educated guess.
In
his intro to Lecture 16, Whittle says, “Different objects make different
sounds, and this is also true for the Universe: If the Universe had different
properties, its primordial sound would be different. Cosmologists have been
extremely successful at measuring many properties of the Universe by comparing
computer calculations of the primordial sound with the sound of the real
Universe. The match is in fact so good that it essentially proves that the Hot
Big Bang Theory is valid and robust.”
Don’t
forget that the CMB radiation is black-body radiation. It’s in the microwave
region of the electromagnetic spectrum today because it’s been redshifted by cosmic expansion from
its original black-body spectrum centered in the infrared region. The infrared photons were what mainly gave
the photon-baryon gas its pressure and temperature, and the small fluctuations
in the density of this gas—the rarefactions and compressions—are what it is
measurable today as former sound waves frozen in time on the microwave background.
These fluctuations tell us when and how the first stars and galaxies formed and
what the universe is made of. We now know the presence of dark matter is necessary
for stars and galaxies to form, but we still don’t know what dark matter is.
By the way, Astronomy magazine's cover story this month is called "Assembling the Universe." So there's an example of how it's better to call even a "simple system" an assembly rather than a system. See my previous post for more on this subject, under the sub-heading "An aside related to vocabulary."
For a short summary of the big bang and its consequences, see this "early universe" website.
For a short summary of what homogeneous means, see this Organic Valley blog post on homogenized milk. A carton of homogenized milk sitting in a refrigerator at 277 K is another example of a simple thermodynamic system in equilibrium.
