Today, understanding dark energy is a subject of importance for Cosmologists, it’s claimed to be the culprit for the accelerated expansion of the Universe.
Understanding dark energy is also the key to understanding the fundamental nature of gravity, and The General Theory of Relativity is the best description of gravity to date.
It works well when observed at the level of galaxies, clusters, black holes, etc, but it is quite off at a larger scale where dark Energy dominates the large-scale structure of the Universe.
In the quest to understand dark energy, a team of 400 Astronomers & Astrophysicists completed the six-year-long Dark Energy Survey and recently released the results of the initial three years of observation1.
25 Institutions from 7 countries supported this collaboration, it’s one of the most extensive surveys of the sky ever conducted, with hundreds of millions of galaxies observed in 1/8th part of the entire night sky, which took them six years.
Dark Energy Survey Overview
Dark Energy Survey, DES was started in August 2013 to investigate the large-scale structure of the universe, this was accomplished using Cerro Tololo Inter-American Observatory in Chile.
This observatory is equipped with the Dark Energy Camera (DECam), it’s one of the highest-resolution optical instruments available in the ground-based observatory.
Its diameter is about 2.2 meters with a 570-megapixel camera and it measures visible light and infrared spectrum.
It surveyed around 5000 square radians of the sky, which took 758 nights in over six years to complete. It took the measurements of more than 300 million galaxies, some of which are around 7 billion light-years away.
Those galaxies are a million times fainter than the faintest star seen by the naked eye, this survey was finally over in January 2019.
On 26th May, they released their first results from the initial data accumulated in the first three years, which created hype among Astronomy & Space enthusiasts.
The results are worth having attention to as they have a lot to tell about the Universe. But before that, let’s see how they conducted the survey.
Science Behind Dark Energy Survey
DES used four probes to observe the Large Scale Structure of the Universe,
- Type Ia Supernova
- Baryon Acoustic Oscillations
- Galaxy Clusters
- Weak Gravitational Lensing
Type Ia Supernova
Type Ia Supernovas are the brightest events in the Universe, these are the Explosions that occur when White dwarf stars start accreting the mass of their companion star in a Binary system.
They can become as bright as an entire galaxy with billions of stars. These are also known as the ‘cosmic candles.’ The luminosity of Type Ia Supernova is similar throughout the Universe.
Hence, their distance can be measured easily by observing their luminosity and redshift. It helps in measuring the distance from a galaxy that contains a type Ia Supernova.
Baryonic Acoustic Oscillations (BAO)
Baryonic Acoustic Oscillation or BAO is the fluctuations in the baryonic matter density.
Baryons are the particles that make up the atoms constituting ordinary matter.
It is caused by the density waves created in the plasma of the primordial Universe, later the plasma cooled down to form neutral atoms, and we can see the aftereffect of the early fluctuations in the form of BAO.
These patterns help astronomers to measure the distribution of the galaxies and the rate of expansion of the Universe.
It also provides the standard ruler for measuring length in the cosmos, just like type Ia Supernova.
Weak Gravitational Lensing
Weak Gravitational Lensing is the minor distortion in galaxies’ images caused by the massive structures in front of it.
Gravitational Lensing is generally categorized into two types – weak gravitational lensing and strong gravitational Lensing.
Strong gravitational Lensing produces a large arc, but it requires information about the composition of the object, which is causing the lensing effect.
But weak gravitational Lensing can help us analyze the foreground distribution without any assumption about its composition.
It might not help make error-free observations, but it can better map the distribution of Dark Matter.
Another probe for the Dark Energy Survey is mapping the distribution of Galaxies through spectroscopy. DECam doesn’t have the best spectroscopy abilities; hence it takes measurements in 5 different wavelengths.
Though these direct observations might not tell the exact distribution of the Dark Energy, combining this data with weak gravitational Lensing would give a better picture of the Dark Matter distribution.
It will also help us to compare the prediction of cosmological theory with the observed data. Our Cosmological models suggest the number of halos that should host the galaxy clusters.
The distribution of these depends upon the expansion of the Universe. This allows us to study the evolution of dark energy as it suppresses the gravitational growth of halos.
The use of these four probes is the key to the whole survey. But why do we need to observe through these four probes? Astronomers will use data obtained from this survey extensively to study different aspects of the Universe’s structure.
So having measurements from four different probes allow Astronomers to research several dimensions. For the same reason, the observations are made accessible to everyone.
Results of the Initial 3 years of DES
Now comes the interesting question, what result do we have from this survey?
So far, astronomers have released the results for only the initial three years of the study, and it has two results to share.
First, they found that our Universe is a bit smoother than what General Relativity has predicted. After a long time, we have some evidence that goes against General Relativity.
But many astrophysicists suggest that it needs some minor corrections instead of scraping it off. After all, General Relativity has performed well in the study of different Cosmological bodies.
It just went a bit off when we studied the large-scale structure. However, it does prove that still, we know very little about the Universe.
Another significant result of this survey is the value of the dark energy equation-of-state parameter denoted by ‘w’. It governs the rate at which the dark energy density evolves; basically, the ratio of pressure exerted and energy density.
It’s found to be around -0.98. If its value is exactly -1, then its Cosmological Constant, i.e., dark energy density remains constant throughout the Universe and is the inherent property of space-time.
As we know, space is expanding, and dark energy is also increasing linearly with it. Due to this increase, our Universe would be expanding at constant acceleration.
If its value is greater than -1, then it would mean that dark energy is some sort of field, and its density would decrease as space will expand. That doesn’t mean that it would vanish, but the acceleration with which Universe is expanding would decrease with time.
But if its value turns less than -1, then it would mean that the expansion of the Universe would undergo exponential acceleration.
If it happens, a time will come when every single atom would be ripped off, and physical matter won’t exist. In previous studies, the value of ‘w‘ has been found closer to -1 most of the time, just slightly less than -1.
For the first time, the value had been found to be slightly greater than -1, i.e., -0.98. So, we might not get shredded apart due to the expanding space.
DES might have given some promising results, but there is still a lot to consider and refine the calculations. These values stand only for 2 sigmas for statistical significance.
It means that the values stand correct 95% of the time. It might seem like a good success ratio, but it must be more accurate to eliminate the chances of instrumental error.
We need more precision when we talk about evidence that goes against one of the most accurate descriptions of the Universe, the General Theory of Relativity.
Hence, it would be early to confirm these values, but it sure gives us hope for finding something significant.
We will have to wait for the DES team to release their full results to understand our Universe better.
- DES Collaboration, ‘Dark Energy Survey Year 3 Results: Cosmological Constraints from Galaxy Clustering and Weak Lensing‘, 2021, https://www.darkenergysurvey.org/wp-content/uploads/2021/05/desy3_3x2_results.pdf