Supernova Remnant SNR 0454-67.2 is believed to be the aftermath of a Type Ia supernova explosion. This conclusion was reached by scientists who studied the light emitted by such supernovae in the late 1990s, leading to the discovery that the universe’s expansion is actually accelerating. The remarkable image of this supernova remnant was captured by the European Space Agency’s Hubble Space Telescope and NASA.
Have you ever wondered if it’s possible to eliminate all forms of energy in the room where you are currently located? Let’s imagine an experiment where we remove the room from the influences of Earth’s gravity. We then proceed to remove all objects made of matter, effectively eliminating mass, kinetic, and potential energies. We take it a step further and pump out the air, removing not only the particles we breathe but also cosmic rays and the fog of neutrinos that were created during the Big Bang. Lastly, we extinguish the energy in photons by creating complete darkness and eliminating microwave radiation remnants from the early stages of the universe. At this point, we might assume that our room is devoid of energy, but that’s not entirely accurate.
Even though it may seem like an empty space, your room still contains something known as “dark energy.” In the specific area occupied by your room, dark energy is so incredibly sparse that its detection becomes nearly impossible. However, on a cosmic scale, where vast expanses of space exist, dark energy contributes a staggering 70% of all energy present. The remaining 30% comes primarily from matter in the form of stars, gas, and the enigmatic dark matter, while radiation, in the form of photons and neutrinos, makes up only a fraction of 0.01%.
It is essential to understand that space is not merely a void of nothingness. Albert Einstein, with his groundbreaking theories, taught us that space is a pliable, stretchable medium that we inhabit, much like fish navigating through water. When energy is evenly distributed across a section of space, that particular area will expand or contract accordingly, depending on whether the energy is positive or negative. Each type of energy contributes to the universe’s expansion in its unique way, similar to how inflating a balloon with air, water, or sand yields distinct characteristics and properties.
Given that dark energy dominates the universe’s energy allocation, it also dictates the rate at which space expands. By reverse-engineering this knowledge and considering the size and age of the universe, we can estimate the amount of dark energy present in any given volume of space. If we were to introduce an excessive amount of positive energy, the universe would expand so rapidly that galaxies would move away from us at a speed faster than light, causing only the regions closest to us to remain visible. Essentially, the observable universe would appear to shrink. Conversely, an abundance of negative energy would result in a universe that contracts and eventually collapses into an infinitesimally tiny point. The magnitude of this negative energy directly influences the speed at which the collapse would occur.
The undeniable truth is that the universe is much larger than India and significantly older than the Indus Valley Civilisation. These facts alone demonstrate the density of dark energy to be comparable to the caloric content found in a mere pinch of sugar within a cubic meter. In reality, the universe extends across billions of light-years and has an age exceeding 10 billion years, rendering dark energy vastly dilute, akin to a single sugar crystal dispersed within a cubic kilometre.
Despite having established a theoretical framework, scientists encounter an alarming problem when attempting to calculate the actual amount of dark energy present in the universe. The simplest estimation suggests that a cube with sides measuring 10^-21 cm should contain enough energy to disassemble the entire Milky Way. However, empirical observations indicate that the actual amount of dark energy is far less than anticipated. This discrepancy presents a formidable challenge, as no convincingly plausible solution has been proposed. In essence, while the universe appears to be astronomically immense, physicists’ calculations suggest that it should be smaller than a proton. This discrepancy, known as the cosmological constant problem, has been rightfully deemed “the worst theoretical prediction in the history of physics.”
To predict the quantity of dark energy solely based on theory, particle physicists rely on a relatively clear understanding of its composition. This contrasts with the mysterious nature of dark matter, which remains largely unknown. Three inescapable factors mimic the behavior of dark energy:
1. The weight of the vacuum: Einstein initially believed that space possessed its own energy, which he referred to as the “cosmological constant.” At the time, physicists assumed that the universe remained static rather than expanding. Consequently, they deemed the cosmological constant unnecessary, and Einstein omitted it from his equations. However, when astronomer Edwin Hubble subsequently revealed the universe’s expansion, Einstein acknowledged his missed opportunity and regretted discarding the cosmological constant, dubbing it his “biggest blunder.”
2. Zero-point energy: Heisenberg’s uncertainty principle, a fundamental tenet of quantum mechanics, stipulates that any physical system possesses a minimum positive energy. This principle applies to quantum fields that generate elementary particles like electrons and photons. Analogous to how sugarcane produces sugar cubes, these fields permeate space and supply energy to every point within the universe.
3. Field potentials: Kinetic energy is inherent in all fields, but certain fields lacking quantum spin, such as the Higgs field that originates the Higgs boson, also possess potential energy. Consequently, these fields contribute energy to every point within the universe.
Contributions 2 and 3 are theoretically calculable and ultimately provide an enormous quantity of energy, capable of reducing the universe’s size to that of a proton. However, the enigma lies with contribution 1, as we have no concrete knowledge about its magnitude. To put it into perspective, imagine attempting to purchase a ship without knowing its cost, offering all of your stocks and real estate, only to receive a fraction of a cent as change. In such a scenario, you would surely suspect the seller of meticulously manipulating the price to the umpteenth decimal place.
The challenge lies in the remarkable fine-tuning of the cosmological constant, which appears to be accurate up to an astonishing 122 decimal places. This degree of precision represents the crux of the problem. Scientists aim to identify the mathematical principle that can reasonably explain this apparent fine-tuning. The potential answers, proposed by influential figures like Stephen Hawking and Steven Weinberg, are just as mind-boggling, but that discussion shall be reserved for another day.
In conclusion, dark energy remains an enigma that defies precise explanation. Despite our best efforts to calculate its presence and amount, we are left grappling with the perplexity of the cosmological constant problem. The universe’s energy composition is a mystifying realm that holds immense potential for scientific discovery, pushing the boundaries of our understanding of the cosmos. As researchers continue to explore and uncover the secrets of dark energy, we inch closer to unraveling one of the greatest mysteries of our universe.