Dark Energy

INTRODUCTION

Almost three-quarters of all energy in the universe is in a mysterious form which we call Dark Energy. Whatever it is, we know its negative energy is dominating the universe and determining its fate. By negative gravity, we mean gravity that pushes things apart, in contrast with normal gravity that always pulls things together. Because of dark energy, the universe will expand forever, and expand at an ever-increasing rate. Some call this the Big Rip.

    The primary evidence for the of dark energy comes from two sources: 

(1) The cosmic microwave background (CMB) radiation CMB

(2) The accelerating expansion of the universe that is observed using Type Ia supernovae

     The Type Ia Supernovae data demonstrates that dark energy began to dominate all other forms of energy about 6 billion years ago. Before then, the attractive gravity of the matter, both normal matter and dark matter, dominated and was gradually slowing down the expansion of the universe. In the last 6 billion years, the negative gravity of dark energy gained the upper hand and is now accelerating the expansion. 

     Physicists have some confidence they know what dark energy is, though we cannot prove it yet. supernovae

DARK ENERGY AND VIRTUAL PARTICLES

      According to Quantum Physics, always and everywhere, particle-antiparticle pairs spontaneously flash into and out of existence. This is a real effect that has been extensively and precisely confirmed. One consequence of virtual particles is that any volume of space, even “empty” space, contains energy. Even though virtual particles lead the briefest of lives, every cubic foot of space will always contain a number of virtual particles. Thus, it’s not unreasonable that the average amount of energy per cubic foot is greater than zero, even in empty space. Not unreasonable, but we don’t yet have an effective physical theory that allows us to calculate the amount of this energy. Our current theories say that the energy of empty space due to virtual particles is absurdly huge---these theories are clearly wrong. A deeper understanding of this issue awaits the completion of a theory called Quantum Gravity that we hope will successfully combine Quantum Mechanics and General Relativity. general relativity

   Meanwhile, it seems inescapable that empty space must have energy. The Casmir Effect (In quantum field theory, the Casimir effect is a physical force acting on the macroscopic boundaries of a confined space which arises from the quantum fluctuations of the field. It is named after the Dutch physicist Hendrik Casimir, who predicted the effect for electromagnetic systems in 1948) is a non-cosmological phenomenon that confirms this and that can be demonstrated in our laboratories. 

    If a certain volume of empty space does have energy E, then it turns out it must have a pressure P; in fact, to ensure energy conservation P = -E. Thus, if the energy of the empty space is greater than zero, its pressure must be negative. General relativity says that a substance with energy and pressure exerts a gravitational force proportional to E+3P. Hence, the gravitational force of empty space should be negative, since:

E+3P = E-3E = -2E < 0

THE BALANCE SHIFTED

    When the universe was younger and smaller, it contained less space and the negative gravity of space was less important. Also, when the universe was smaller, the density of matter (both normal and dark) was higher, and matter’s positive energy was more important.

     Matter’s positive gravity exceeded dark energy’s negative gravity, and although the Universe continued expanding, its expansion rate was gradually decreasing. But, as space expanded, the balance shifted. The ever-increasing volume of space led to an ever-increasing amount of dark energy and also to an ever-decreasing density of matter. A time came about 6 billion years ago, when dark energy began to dominate.

    As space continued to expand, the density of matter will drop further and the amount of space, and the amount of dark energy, will increase even more. The global expansion of the universe will become exponential, increasing more rapidly, without limit.

    But galaxy clusters, and the Local Group in particular, will remain together due to the strength of their mutual normal gravity. As long as galaxies in the Local Group don’t separate, the amount of space between them will not increase, neither will the amount of dark energy in this region.

IS DARK ENERGY THE COSMOLOGICAL CONSTANT?

   Is dark energy the reincarnation of Einstein’s “greatest blunder”, many physicists would say yes and applaud Einstein for predicting dark energy 80 years prior to its discovery.

    But this is not really what it seems. There are two important differences between dark energy and Einstein’s cosmological constant. The first key difference is the physical effect. Einstein introduced his cosmological constant to counterbalance the gravitational attraction of matter and provide static solution to his Field Equations. Dark energy provides neither balance nor a static solution; it is relentlessly driving the universe ever further away from balance and stability. The effect of dark energy is nothing like what Einstein sought to achieve.

   Secondly dark energy and cosmological constant differ in methodology and philosophy. When Einstein modified his Field Equations in 1917, he added the cosmological constant to the geometry side of his equations, making them:

                            G + Λ = 8ᴨT

 This means the effect is somehow due to the geometry of our universe. Energy, in all of its forms, is properly on the right-hand side of the equation included in T:

    G = 8ᴨ ( Tmat + Trad+ TDark M + TDark E )

Here, the terms on the right represents the contributions to total T from normal matter, radiation, dark matter, and dark energy. Of course, it ultimately makes no computational difference whether one adds something from the left or one subtracts something from the right. But inserting an unknown geometry effect is very different philosophically from including the energy of virtual particles due to quantum nature of the micro world. The first is an unexplained fudge factor, whereas the second is a working hypothesis with a solid physical motivation.

   There is no need to find false reasons to praise Einstein. His many questionable, outstanding achievements certainly place Albert Einstein with Sir Isaac Newton as two greatest scientific minds of all time. People can debate which one was greater, but these two stand head and shoulders above all others is beyond debate.

    That each was sometimes wrong merely adds a touch of humanity.

.                            Image credit- google

Article by- 

Saptashaw Das