Measuring the Cosmos: How Astronomers Determine Distances & The Observable Universe vs. The Actual Universe

Measuring cosmic distances requires ingenuity, as we can't simply stretch a tape measure to the stars. Astronomers use a "cosmic distance ladder," where each method builds on the previous one, reaching ever further into space.

For nearby objects, we use parallax – the apparent shift in position when viewed from different locations. Hold your finger at arm's length and alternately close each eye; your finger appears to move against the background. Earth's orbit provides a baseline of 300 million kilometers, allowing us to measure stellar parallax for stars up to a few thousand light-years away. The European Space Agency's Gaia mission has measured parallax for over a billion stars.

For more distant stars, we use "standard candles" – objects with known brightness. Cepheid variables are pulsating stars whose period relates to their true brightness. By comparing their apparent brightness to their calculated true brightness, we can determine distance. Henrietta Swan Leavitt discovered this relationship in 1908, revolutionizing astronomy.

Type Ia supernovae serve as even brighter standard candles, visible across billions of light-years. These stellar explosions have consistent peak brightness, making them cosmic lighthouses. By measuring their apparent brightness and light spectrum, we can determine both distance and recession speed, revealing the universe's expansion.

For the furthest galaxies, we rely on redshift – the stretching of light waves due to cosmic expansion. The faster a galaxy recedes, the more its light shifts toward red wavelengths. Hubble's Law relates recession velocity to distance, though dark energy complicates this at extreme distances. The most distant galaxies show such extreme redshift that their visible light has shifted into the infrared.

Each step up the distance ladder involves careful calibration and cross-checking, building a coherent picture of cosmic scales.

Here's a mind-bending fact: the universe has a horizon. We can only see objects whose light has had time to reach us since the Big Bang 13.8 billion years ago. This creates an "observable universe" – a sphere centered on Earth containing everything we can possibly see or detect.

You might think the observable universe has a radius of 13.8 billion light-years, but it's actually much larger. Due to cosmic expansion, objects whose light has traveled for 13.8 billion years are now much farther away. The current distance to the edge of the observable universe is about 46.5 billion light-years, giving a diameter of 93 billion light-years.

This observable universe contains an estimated 2 trillion galaxies, each with hundreds of billions of stars. Yet this might be a tiny fraction of the whole universe. Inflation theory suggests the actual universe could be vastly larger – perhaps infinite. We're like an ant on a beach ball, able to see only a tiny patch of surface while the full sphere extends far beyond our horizon.

The cosmic horizon also means we see objects as they were, not as they are. The farther we look, the further back in time we see. The most distant galaxies appear as they were over 13 billion years ago, when the universe was young. We literally look back in time as we peer deeper into space, making telescopes time machines.

This raises profound questions. What lies beyond our cosmic horizon? Are there other regions with different properties? Could there be other observable universes centered on distant locations? The universe might be far stranger and larger than even our vast observable portion suggests.