What Is Dark Matter? by Peter Fisher

Author:Peter Fisher
Language: eng
Format: epub
Publisher: Princeton University Press
Published: 2022-04-19T00:00:00+00:00

Figure 4.1. Lensing transiting objects in the Milky Way. A) An observer on Earth viewing a star in the LMC would see the star magnified by up to 30% if a gravitating object were to pass within one Einstein radius of the line of sight connecting the star to the observer. B) The change in the brightness of the distant star as the lensing object passes with time. For typical values of the masses and distances, the transit time would be several days.

The solution is to look at a large collection of stars like the LMC. The MACHO collaboration made use of a 1.4-meter telescope in Australia to take repeated electronic images of the LMC each night over nearly 6 years. They sampled a total of 33 million stars, of which 18 million were analyzed in detail. For a given star, a plot giving the brightness of the star as a function of time was made over 5 years of observing. A star magnified by a passing MACHO would get up to 30% brighter when the Einstein radius of the MACHO crossed the line of sight (see Fig. 4.1). A MACHO moving at 200 km/s would take about 3 days to travel one Einstein radius, so the MACHO collaboration scanned their light curves for transit times ranging from 1 to 30 days.

MACHO and later similar experiments—Expérience pour la Recherche d’Objets Sombres (EROS, or “Experiment to Find Dark Objects”), Optical Gravitational Lensing Experiment (OGLE), Andromeda Gravitational Amplification Pixel Experiment (AGAPE), and others—observed many new kinds of variable stars (stars whose brightness varies with time, owing to their internal dynamics). These new variable stars made data analysis difficult. Also, about half of all stars are part of binary systems consisting of two objects orbiting each other, causing a variation in the observed brightness of the system. Both binary objects and variable stars had to be identified and filtered out. With about 18 million stars to look at over 6 years, and a probability of 2 × 10−5 transits per year per star, MACHO experiment collaborators expected several dozen transits if dark matter were composed of MACHOs. Early on, the MACHO survey observed about a dozen events whose transit times indicated that the lensing objects had a mass of around half a solar mass. This transit rate would correspond to MACHOs accounting for about 20% of the galaxy’s dark matter. However, subsequent measurements by MACHO, EROS, and OGLE experiments found far fewer transits, leading to the conclusion that dark matter cannot be massive baryonic objects the size of our sun. The first measurement was a statistical fluke.

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