If dark energy in the universe currently is about 73 % of...

If dark energy in the universe currently is about $73 \%$ of the critical density, then how many joules of energy are there in each liter of space? How many 1 -mm-wavelength microwave photons would this correspond to in each liter?

Key Concepts

Unit Conversions

Unit conversions are essential in physics when relating theoretical quantities to practical measurements. In the context of this problem, converting energy density expressed over a large volume (such as per liter) into the equivalent number of microwave photons involves understanding both the energy per photon and the conversion factors between the different units of energy and volume.

Photon Energy

Photon energy is determined by the wavelength or frequency of the photon, where shorter wavelengths correspond to higher energy. This relationship is given by Planck's equation (E = hc/?), which allows one to calculate the energy of a single photon based on its wavelength, such as the microwave photons mentioned in the question.

Dark Energy

Dark energy is a mysterious form of energy that is hypothesized to permeate all of space and is responsible for the observed acceleration in the expansion of the universe. It constitutes the majority of the energy density in the universe, often expressed as a percentage of the total critical density, and its precise nature remains one of the biggest unsolved problems in cosmology.

Critical Density

Critical density is the theoretical density of matter and energy in the universe that would result in a flat geometry. It serves as a key parameter in cosmological models, determining the overall fate and geometry of the universe, and is used as a benchmark to measure contributions from various components such as dark energy and matter.

Energy Density

Energy density refers to the amount of energy stored in a given volume of space. In the context of cosmology, it is often expressed in joules per liter or joules per cubic meter, allowing researchers to relate the large-scale energy content of the universe, including dark energy, to more familiar physical units used in laboratory settings.

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