Ashoke Sen discusses the classical decay of unstable D-branes in bosonic and superstring theories, showing that it produces a pressureless gas with non-zero energy density. The energy is stored in open string fields, even though the tachyon potential's minimum has no open string degrees of freedom. This phenomenon is described in an effective field theory. In bosonic string theory, the energy-momentum tensor for the rolling tachyon solution approaches a pressureless gas with constant energy density as the tachyon field rolls towards its minimum. In superstring theories, similar results are found for unstable D-branes and brane-antibrane configurations. The energy density remains constant while pressure approaches zero. The analysis uses boundary states and effective field theory to describe the tachyon matter system, which could potentially play a role in cosmology, such as contributing to dark matter. The study also explores tachyon condensation in other unstable brane systems and its implications for supersymmetry breaking. The results are consistent across bosonic and superstring theories, showing that the system evolves to a pressureless gas as the tachyon field approaches its minimum. The effective field theory analysis confirms these findings, showing that the energy-momentum tensor for the tachyon field approaches a pressureless gas with constant energy density. The study concludes that the tachyon matter system has potential applications in cosmology and provides insights into the behavior of tachyon condensation in various brane systems.Ashoke Sen discusses the classical decay of unstable D-branes in bosonic and superstring theories, showing that it produces a pressureless gas with non-zero energy density. The energy is stored in open string fields, even though the tachyon potential's minimum has no open string degrees of freedom. This phenomenon is described in an effective field theory. In bosonic string theory, the energy-momentum tensor for the rolling tachyon solution approaches a pressureless gas with constant energy density as the tachyon field rolls towards its minimum. In superstring theories, similar results are found for unstable D-branes and brane-antibrane configurations. The energy density remains constant while pressure approaches zero. The analysis uses boundary states and effective field theory to describe the tachyon matter system, which could potentially play a role in cosmology, such as contributing to dark matter. The study also explores tachyon condensation in other unstable brane systems and its implications for supersymmetry breaking. The results are consistent across bosonic and superstring theories, showing that the system evolves to a pressureless gas as the tachyon field approaches its minimum. The effective field theory analysis confirms these findings, showing that the energy-momentum tensor for the tachyon field approaches a pressureless gas with constant energy density. The study concludes that the tachyon matter system has potential applications in cosmology and provides insights into the behavior of tachyon condensation in various brane systems.