Cosmos expands by creating space or stretching?

There is overwhelming evidence that the Cosmos is expanding. But how? By creating new space altogether or by stretching the existing space?

Almost 100 years have passed since humanity first reached a revolutionary inference about the nature of our Cosmos: universe itself cannot and does not remain static, but instead evolves with the passage of time. One of the most disconcerting predictions of Einstein’s general relativity is that any Universe — so long as it’s evenly (or almost evenly) filled with one or more than one species of matter, energy, orradiation — cannot stay the same over time. Instead, it must either contractorexpand, something initially derived independently by three different people: Alexander Friedmann in 1922, Georges Lemaitre in 1927, and Howard Robertson in 1929, and was later on generalized by Arthur Walker in 1936.

Around the same time, beginning in 1923, observations began to show that the ellipticalsand spirals in our sky were in fact galaxies: “island universes” that were well outside of our own galaxy Milky Way. With new, more authoritative measurements, we could conclude that the farther a galaxy was from us, greater the arriving light in our devices was redshifted, or observed at lengthier wavelengths, compared to the light that was initially emitted. It was as if the very act of traveling through space altered the wavelength of that traveling light.

But what, precisely, happens to the fabric of space itself while this process transpires? Is the space itself stretching, as though it’s getting stretched outand thinner? Is more space continually being created, as though it were “filling in the gaps” that the expansion fashions? This is among the toughest things to conceive of in modern astrophysics. If we think hard about it, nonetheless, we’re capable of wrapping our heads around it.

All of the radiation within our Cosmos, including every single gravitational wave, behaves as though space is stretching, although space itself isn’t getting any thinner. The dark energy within our Cosmos behaves as though new space is getting created all the time, although there’s absolutely nothing we can measure to detect this creation. In reality, general relativity can merely tell us how space behaves, evolves, and affects the energy within it; it cannot fundamentally tell us what space really “is” in a philosophical sense.

Universe responds in a fundamentally different way to differing forms of energy. If you contemplate a Universe filled with radiation, because the wavelength extends as the Universe expands, the “space stretches” analogy works very well. When something stretches in a conventional manner, it thins out, just as when something compresses, it thickens up. This is a realistic line of thought for describing the conduct of light and other forms of radiation, but not for dark energy. When we consider dark energy, the energy density remains constant permanently. As the Cosmos expands, its volume is growing while the energy density doesn’t change, and hence the total energy increases. From the perspective of dark energy, it’s as if new space is getting created due to the Cosmos’s expansion.