The Cosmological Constant
The cosmological constant is connected with the accelerated expansion of the universe. Its observed value is very small compared with values that might be expected from some theories of particle physics.
The Investigative Question
Why does the cosmological constant have a value that allows matter to gather into galaxies, stars, and planets?
What We Observe
Our universe is expanding, and that expansion is accelerating. A small positive cosmological constant is one way of describing the energy associated with empty space that drives this acceleration.
The value matters. If this effect had been much stronger during the period when galaxies were forming, matter would have been pulled apart too quickly for gravity to gather it into large structures.
Scientific Background
Albert Einstein introduced the cosmological constant into his equations of general relativity. Today it is again an important part of cosmology because observations show that the expansion of the universe is accelerating.
The deeper puzzle is that theories of particle physics can suggest contributions to the energy of empty space far larger than the value inferred from cosmological observations. Explaining this difference is known as the cosmological constant problem.
Why It Matters
Without galaxies and long-lived stars, there would be no known setting in which planets and complex life could develop. The cosmological constant therefore connects one of the deepest problems in theoretical physics with the conditions needed for a life-permitting universe.
What Is Known
The universe is undergoing accelerated expansion. A cosmological constant, or something behaving very much like it, can account for that acceleration.
A sufficiently large positive cosmological constant would interfere with the formation of gravitationally bound structures such as galaxies.
What Is Proposed
Several explanations have been proposed for the small observed value. These include a deeper law of physics, mechanisms that reduce or cancel vacuum energy, and selection effects if many universes or regions with different values exist.
Intelligent Design considers whether a life-permitting value may instead, or also, be evidence of purposeful selection.
What Remains Uncertain
Physics does not yet have an agreed explanation for why the cosmological constant has its observed value. It is also uncertain whether what we call dark energy is truly a constant or reflects some more complicated feature of the universe.
Key Numbers
Observed dark-energy density is extraordinarily small compared with some theoretical estimates of vacuum-energy contributions. A sufficiently large positive cosmological constant would prevent gravitationally bound structures from forming.
Design Relevance
The cosmological constant is often included in arguments for cosmic fine-tuning because its value affects whether galaxies can form. The design question is whether its life-permitting value is better explained by purpose than by physical necessity, selection effects, or some still-undiscovered natural mechanism.
Assessment
The underlying cosmological problem is real. However, claims that assign a precise probability to the observed value require assumptions about what other values were possible and how likely those values would have been. Those assumptions should be examined separately from the observation itself.