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## Imam PBUH Facebook Post: Dark Energy

- 23 February 2014
- Super User

In the name of Allah, the Abundantly Merciful, the Intensely Merciful.

Praise be to Allah Lord of the worlds.

May the peace, mercy, and blessings of Allah be upon you.

Dark energy:

In the twentieth century, astronomical observations of Type Ia supernova — which explodes when a white dwarf reaches 1.4 solar masses — provided cosmological standard candles by which it is possible to know astronomical distances accurately. This is because when this type of white dwarf explodes and transforms into a supernova, they all produce the same luminosity and dim at the same rate since they all have a nearly identical composition. Additionally, they all explode when reaching the same 1.4 solar masses, which represents the white dwarf’s maximum mass, as it strips the hydrogen-rich gases from its aging companion star. As such, density and heat continuously increase until the temperature reaches more than 10 million degrees and nuclear fusion of the entire white dwarf occurs. The star then ignites and explodes massively, tearing the star apart and producing a supernova that is greater than Type Ia.

What causes the increase or decrease in the brightness of the white dwarfs is the distance between them and the observer, or the distance between the supernova and the observer. This is what makes them standard candles for accurately determining astronomical distances, as mentioned before. For example, if we know the distance between us and a supernova, and if we want to measure the distance between us and a second supernova that has a quarter of the first supernova’s brightness, then the distance between us and the second supernova is twice the distance between us and the first supernova. This is because brightness is proportional to the square of the distance. This also means that if we know the distance between us and a supernova we can calculate its luminosity. Since supernovae are continuously exploding in the universe around us, they have provided accurate information about astronomical distances. In addition, observing them showed the speed of the expansion of the universe (matter and energy).

At the end of the twentieth century, a team of researchers who observed supernovae concluded that a supernova far from us is less bright than it should be. This means that the universe is expanding at a speed greater than expected, which in turn means there is increasing, massive, unknown energy resisting the gravity of the mass of cosmic matter and pushing toward expansion at an increasing rate.

Following the development of a tool for measuring precise astronomical distances, and by measuring distances of galaxies and the speed of their distancing, astronomers discovered that there is great unknown energy fighting the gravitational force of matter in the universe, and actively partaking in the continuous expansion of the universe at an increasing rate. This energy was called dark energy.

As for the mathematical calculation of this issue, depending on the above and available results and observations, astronomers were able to know the value of the difference between ΩΛ - ΩΜ = 0.46 ± 0.03.

ΩΛ represents the ratio of the density provided by dark energy to the critical density.

ΩΜ represents the ratio of the average density of all matter in the universe to the critical density.

The critical density is the density at which the curvature of the universe is zero, according to Einstein’s equations.

According to the results of astronomical observations, in the visible universe, the ratio of the average density of all matter including dark matter in the universe — calculated based on gravity — to the critical density is almost 0.25, meaning ΩΜ ≈ 0.25.

From the equation above, we can find that the value of ΩΛ is:

ΩΛ = ΩΜ + 0.46

ΩΛ = 0.46 (±0.03) + 0.25 ≈ 0.71

This means that ΩΛ + ΩΜ = 0.96~0.99.

To some physicists and astronomers, this number is almost one, meaning the curvature of the universe is zero.

From Einstein’s relativity equation about the shape of the universe and its expansion or stability, we can know the value of the critical density of matter in the universe. Critical density is the density of matter in the universe at which the curvature of space is zero. Actual density is the density of the universe that is actually measured, including the cosmic energy converted to the matter that it equals according to Einstein’s equation E=mc2.

If the actual density is larger than the critical density, the curvature of the universe is positive like the surface of a sphere. This also means that if our universe is expanding, it will eventually contract and its expansion will not continue forever.

If the actual density is less than the critical density, this means that the curvature of the universe is negative, like a hyperbolic paraboloid or the saddle of a horse, and its expansion will continue forever.

If the value of the actual density is equal to the value of the critical density, the curvature of the universe is zero, or let’s say it is flat, and its expansion will continue. However, the rate of its expansion will slow down and approach zero without ever actually reaching it.

According to the previous results, the ratio is nearly 1. This means that the actual density is equal to the critical density, meaning the curvature of the universe is zero and it is flat.

In addition to what supernovae have provided, as mentioned above, observation of cosmic background radiation and the drawing of a precise map of the deviations within it using sophisticated equipment mounted on aircraft and satellites at the end of the twentieth century and beginning of the twenty-first century enabled scientists to determine the sum of ΩΛ and ΩΜ, and the value was almost 1.02 ± 0.02. If we go back to the result provided by supernovae of the value of the difference between them we would obtain almost the same result, which is that their sum is almost 1.

This means the cosmological constant in Einstein’s equation is not 0 as previously expected, and there is an unknown energy, or dark energy, that represents the majority of the energy affecting the universe, and if converted it to mass, it is the greatest contributor to the mass of the universe.

The other issue is that the curvature of the universe is 0, meaning that it is flat.