Rather, some contrary component causing the Universe to expand faster must be added to make sense of the observations.
From the point of view of the equations of general relativity, such an additional component is described by the cosmological constant, with a positive value of approximately 0. A cosmological constant multiplying the metric tensor was introduced into general relativity by Einstein in in order to define a system of equations with a static, homogeneous, and isotropic solution — such as he believed was necessary at least as an idealization of the actual universe. The discovery of dark energy has dramatically changed the situation, but it need not signal a change to the law of gravity: it can be understood as a special type of energy-momentum density, to appear on the right-hand side of the field equations along with other sources of energy-momentum density.
Another candidate is that it is the zero-point energy of quantum fields, the existence of which is demonstrated in the Casimir effect. That, in turn, would explain why expansion rates disagree. Dark energy is the unknown, mysterious form of energy that permeates space, flinging the universe outward at faster and faster speeds.
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But in the past two decades, scientists who study the universe's accelerating expansion have found two very different rates. The universes' first light — the cosmic microwave background radiation or CMB — suggests a lower rate for the expansion of space than do studies of supernovas and pulsating stars in the nearby universe.
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In other words, the universe seems to be expanding faster now than would be predicted by how it looked in the early history, shortly after the Big Bang. This disagreement has been termed the " Hubble tension.
A new paper, published June 4 in the journal Physical Review Letters , proposes that early dark energy could be the missing piece that altered the universe's early expansion rate. If so, this early dark energy would have subtly affected the way that CMB looks, explaining why the measured expansion is lower than expected. Future high-resolution observations of the CMB might be able to show if early dark energy really did exist in the young universe.