Phantom energy

In cosmology, phantom energy or phantom dark energy^[1] is a hypothetical form of dark energy. It possesses negative kinetic energy, and predicts expansion of the universe in excess of that predicted by a cosmological constant, which leads to a Big Rip. The idea of phantom energy is often dismissed, as it would suggest that the vacuum is unstable with negative mass particles bursting into existence.^[2] The concept is hence tied to emerging theories of a continuously created negative mass dark fluid, in which the cosmological constant can vary as a function of time.^[3][4] It is a special type of quintessence.

The term was coined by Robert R. Caldwell in 1999.^[5][6]

Equation of state

In cosmology, the equation of state of a perfect fluid is given by

$${\displaystyle p=w\rho ,}$$

where p is the pressure, ρ is the energy density and w is the ratio between the two. For normal baryonic matter ${\displaystyle w=0}$ and for a cosmological constant ${\displaystyle w=-1}$. Phantom energy is defined as having ${\displaystyle w<-1}$.^[1]

Big Rip mechanism

Main article: Big Rip

The existence of phantom energy could cause the expansion of the universe to accelerate so quickly that a scenario known as the Big Rip, a possible end to the universe, occurs. The expansion of the universe reaches an infinite degree in finite time, causing expansion to accelerate without bounds. This acceleration necessarily passes the speed of light (since it involves expansion of the universe itself, not particles moving within it), causing more and more objects to leave our observable universe faster than its expansion, as light and information emitted from distant stars and other cosmic sources cannot "catch up" with the expansion. As the observable universe expands, objects will be unable to interact with each other via fundamental forces, and eventually, the expansion will prevent any action of forces between any particles, even within atoms, "ripping apart" the universe, making distances between individual particles infinite.

One application of phantom energy in 2007 was to a cyclic model of the universe, which reverses its expansion extremely shortly before the would-be Big Rip.^[7] This cyclic model can be more complicated if the mass–energy of every point in the universe is dense enough to collapse into black hole core substance that will bounce after reaching a maximum threshold of compression causing the next big bang (the overall scenario is highly unlikely).

Possible evidence

In 2025, the Dark Energy Spectroscopic Instrument (DESI) collaboration, published a survey on baryon acoustic oscillations. They found violations of the standard model of cosmology, the Lambda-CDM model, within 4 standard deviations. They reported acceleration of the universe that was stronger in the past, suggesting the presence of phantom energy in the early universe.^[8]