# Hubble Tension

The Hubble tension refers to a significant discrepancy in cosmology between different methods of measuring the Hubble constant, which describes the current expansion rate of the universe. Early universe measurements based on the cosmic microwave background predict a Hubble constant of approximately 67.4 kilometers per second per megaparsec (km/s/Mpc), while direct measurements in the local universe consistently indicate roughly 73.2 km/s/Mpc.

## Overview

The Hubble tension represents a significant challenge to modern cosmology because both measurement approaches are considered robust and well-tested. The cosmic microwave background (CMB) data from the European Space Agency's Planck spacecraft shows the state of the universe approximately 379,000 years after the Big Bang, predicting a specific expansion rate based on the Lambda-CDM model.

## Measurement Methods
### Early Universe Measurements

The primary measurement of the early universe comes from observations of the cosmic microwave background radiation. Before 379,000 years after the Big Bang, the universe was filled with an opaque plasma. As it cooled, electrons and nuclei combined to form neutral atoms, allowing light to travel freely. The CMB captures the pattern of density waves (baryonic acoustic oscillations) from this early period.

### Local Universe Measurements

Direct measurements in the nearby universe use various "standard candles" to establish cosmic distances:
* Cepheid variable stars
* Type Ia supernovae
* Red giant stars
* Gravitational wave events with electromagnetic counterparts

## Recent Developments

In early 2025, research led by Dan Scolnic at Duke University found that the Coma Cluster is approximately 320 million light-years from Earth, significantly closer than the 359 million light-years predicted by the standard model. This finding brought the Hubble tension into our cosmic "backyard" and strengthened the case that the discrepancy represents a real crisis in cosmology rather than a measurement error.
The Dark Energy Spectroscopic Instrument (DESI) has provided additional evidence for the tension, measuring different values for the Hubble constant depending on whether it uses early or late universe methods. When using observations of baryonic acoustic oscillations in galaxy distribution, DESI calculates a value of 68.5 km/s/Mpc, close to the Planck prediction. However, when measuring the local universe using Type Ia supernovae, it finds a value of approximately 76 km/s/Mpc.

## Proposed Resolutions

Several theoretical solutions have been proposed to resolve the tension:
* Modified dark energy behavior in the early universe
* Additional particle species affecting early cosmic evolution
* Modifications to the standard cosmological model
* Potential systematic measurement errors
However, extensive work by both the Hubble Space Telescope and James Webb Space Telescope teams has ruled out many potential systematic errors in the measurements.

## Future Research

Several upcoming projects may help resolve the tension:
* The Nancy Grace Roman Space Telescope will conduct wide celestial surveys to study dark energy's influence
* The Euclid observatory, with NASA contributions, is pursuing similar investigations
* Continued observations by the James Webb Space Telescope may provide additional insights