Does Dark Energy Have Gravity?

Yes—but “gravity” needs a little context here. In general relativity, gravity is not sourced only by ordinary matter. Energy density and pressure also contribute. Dark energy therefore gravitates, and for the simplest model, the cosmological constant, its effect on the expansion of the universe is to make that expansion accelerate.

That does not mean dark energy behaves like a nearby object pulling on you, or like a force that pushes galaxies apart locally. Its best-understood effect is on the large-scale evolution of space.

How energy and pressure gravitate

In cosmology, the acceleration of the universe’s scale factor is described by the acceleration equation. If the cosmic contents are represented as fluids, it can be written as

$$\frac{\ddot a}{a}=-\frac{4\pi G}{3}\left(\rho+\frac{3p}{c^2}\right),$$

where $a$ is the scale factor, $G$ is the gravitational constant, $c$ is the speed of light, and $\rho$ and $p$ are the total mass-equivalent density and pressure of the cosmic contents. Here, $\rho$ means energy density divided by $c^2$.

The combination $\rho+3p/c^2$ shows why pressure matters. For ordinary matter, pressure is usually small compared with energy density, so matter contributes to slowing expansion. Dark energy has sufficiently negative pressure that its contribution to this combination is negative. A negative right-hand side becomes a positive $\ddot a$: expansion accelerates.

The cosmological-constant example

The simplest model of dark energy is the cosmological constant, often written $\Lambda$. It has constant density and pressure

$$p_\Lambda=-\rho_\Lambda c^2.$$

Its contribution to the acceleration equation is therefore

$$-\frac{4\pi G}{3}\left(\rho_\Lambda+\frac{3p_\Lambda}{c^2}\right) = -\frac{4\pi G}{3}(\rho_\Lambda-3\rho_\Lambda) = \frac{8\pi G}{3}\rho_\Lambda > 0.$$

So the answer is not that dark energy “has no gravity.” Rather, its energy and pressure affect spacetime in a way that accelerates cosmic expansion. In general relativity, “repulsive gravity” is sometimes used as shorthand for this outcome. It should not be taken to mean that dark energy is an ordinary repulsive force acting between nearby objects.

A numerical example

A representative present-day mass-equivalent density for dark energy is about $6\times10^{-27}\ \mathrm{kg/m^3}$. Its contribution to the acceleration rate of the scale factor is approximately

$$\frac{8\pi G}{3}\rho_\Lambda \approx 3.4\times10^{-36}\ \mathrm{s^{-2}}.$$

For an ideal separation of one megaparsec ($1\ \mathrm{Mpc}\approx3.1\times10^{22}\ \mathrm{m}$) that follows the overall cosmic expansion, multiplying this rate by the separation gives an acceleration contribution of roughly $1.1\times10^{-13}\ \mathrm{m/s^2}$. This is a cosmological term in the evolution of that separation, not a universal extra force to add to every local calculation. The small size helps explain why dark energy is not noticeable in everyday or solar-system dynamics.

Does it attract or repel?

The word “gravity” is often used to mean the attractive pull between masses. Under that narrower, everyday meaning, dark energy does not behave like matter: its pressure is negative, and its main gravitational effect on large scales is associated with accelerated expansion rather than extra attraction.

But in the broader and more accurate general-relativistic sense, dark energy does gravitate. The expansion history depends on its density and pressure. This is true whether dark energy is a cosmological constant or a more complicated, changing component—though the detailed equations of state may differ.

Why galaxies and solar systems stay bound

The cosmological effect is extremely weak on small scales. Systems such as the Solar System, stars, and galaxies are bound by local gravity and other forces; they do not simply expand along with the universe. For the cosmological constant, its effect grows with distance in the idealized description of cosmic expansion, but at the scale of a planetary system it is tiny compared with the Sun’s gravitational influence.

Dark energy is also observed through its influence on the universe as a whole, not as a substance that has been directly collected or measured in a laboratory. The cosmological constant is a successful simple model, while the underlying nature of dark energy remains an open question.

Common misunderstandings

• “Only mass creates gravity.” In general relativity, energy and pressure contribute too.
• “Accelerated expansion means dark energy pushes objects apart locally.” It describes the large-scale expansion of the universe; bound systems do not expand in that simple way.
• “Negative pressure means negative energy.” For a cosmological constant, the energy density is positive while the pressure is negative.
• “Dark energy has no gravitational effect because it is uniform.” Uniformity means it does not clump like ordinary matter, not that it is absent from the equations governing cosmic expansion.