Understanding Redshift in Cosmology: Determining Original Wavelengths

When discussing cosmological redshift, it's crucial to understand how astronomers determine the original wavelengths of light from celestial bodies. This article will explore the methods and principles used by scientists to identify redshift, effectively solving the conundrum of how we know the original wavelengths of light from distant stars.

What is Cosmological Redshift?

Cosmological redshift is a phenomenon where the light from distant galaxies appears to have shifted towards the red end of the spectrum, indicating that the universe is expanding. However, this does not mean that we can directly measure the original wavelength of the light from these distant stars. Instead, we use specific techniques to deduce the shift and its implications.

Identifying Elements in Stars

A key aspect of understanding redshift involves examining the spectrogram of a star. Spectrograms reveal the frequencies of light absorbed by certain elements in the star's atmosphere. By comparing these frequencies to known values, we can identify the elements present, such as hydrogen and helium, and understand their behavior.

Using Spectroscopy to Identify Elements

When we observe a spectrogram, we see black bars at specific frequencies. These frequencies correspond to the absorption lines of different elements. For example, hydrogen and helium have distinct absorption patterns that are consistent across the universe. If these lines appear in a different position compared to known references, it indicates a redshift or blueshift.

To determine the redshift, scientists often compare the spectrogram of a distant star to the Sun. By aligning the spectra and sliding them to match, the discrepancy in position shows the shift in wavelength. If the pattern shifts towards the red end, it is redshifted. This comparative method allows us to identify changes in wavelength without knowing the exact original wavelength of the light.

The Role of Doppler Effect

The Doppler effect is a crucial factor in understanding redshift. This effect occurs when the source of light is moving relative to the observer, causing the observed wavelengths to shift. The same elemental absorption patterns appear in the wrong position due to this effect. By noting the new frequencies and comparing them to unshifted frequencies from Earth-bound samples, scientists can confirm and quantify the Doppler shift.

Examples and Verification

One practical example is observing sodium street lamps or gas flames turning yellow due to the emission and absorption of sodium light. The characteristic frequencies of these emissions are consistent and unchanging, allowing us to recognize the shifts. The same applies to other elements in stars; the shift in the spectral lines provides clear evidence of redshift or blueshift.

Astronomers often use known patterns of elemental absorption lines to verify redshift. By lining up the spectrum of a distant star with the Sun's, they can see how the absorption lines have shifted. If the lines are shifted towards the red end, the star is receding; if towards the blue end, it is approaching. This method is robust and consistent, providing clear evidence of redshift.

In summary, determining the original wavelengths of light from distant stars is not about measuring the exact wavelengths of the light itself, but rather comparing the characteristic absorption patterns of elements in the star’s spectrum to known patterns on Earth. This approach allows scientists to accurately identify and measure redshifts, providing valuable insights into the expansion of the universe and the behavior of distant celestial bodies.