This article is from
Journal of Creation 36(2):12–14, August 2022

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Central rotation in globular clusters: an indicator of relative youth?

by Jake Hebert

Image: NASA and ESA.

Globular clusters are beautiful, spherically symmetric star clusters resembling celestial snow globes. Star density is greatest near the cluster centre, and this density drops off with increasing distance from the cluster centre. Approximately 150 globular clusters orbit our own Milky Way galaxy, including the Messier 80 globular cluster in the constellation Scorpius (figure 1).

Although they do not understand how or where globular clusters formed,¹ uniformitarian astronomers have long claimed that globular clusters are among the oldest objects in the universe, with typical ages greater than 10 Ga. In fact, uniformitarian astronomers sometimes define globular clusters as old star clusters found in the bulges and halos of galaxies.² Recently, some uniformitarian astronomers have suggested globular clusters might be four billion years younger than previously claimed.^3,4 But in any case, by secular reckoning, their ages are measured in multiple billions of years, and it is generally agreed that the globular clusters orbiting our own Milky Way galaxy have ages of at least 11 Ga.⁵

The well-known ‘neutron star retention problem’ is the presence of higher-than-expected numbers of neutron stars in globular clusters. This is a possible indicator that globular clusters are much younger than claimed by secular scientists, and it has already been discussed in the creation literature.⁶

Rotational signatures

Eight years ago, another possible indicator of the relative youth of globular clusters appeared in both the popular science press and in the technical literature.^7,8 Astronomers affiliated with the University of Texas and the Max Planck Institute for Extraterrestrial Physics found evidence, via line-of-sight kinematic studies, that stars in the cores of 11 Milky Way globular clusters were orbiting a preferred axis of rotation. Astronomers have already detected rotational signatures in the outer regions of some nearby well-known globular clusters. However, they were surprised to see such a rotational signature in the central parts of these clusters.

According to textbook theory, stars in globular clusters exert gravitational ‘tugs’ on one another as they undergo 2-body interactions. These tugs nudge the orbiting stars, changing their trajectories little by little. Given enough time and enough interactions, these gravitational tugs will eventually ‘erase’ any ‘memory’ of the stars’ original orbits, completely randomizing their trajectories. The time for this effect to become significant is called the relaxation time. Depending on the method of derivation, formulae for the relaxation time can vary somewhat, but a common expression⁹ for the relaxation time is:

In the above equation , ⟨v²⟩ is the average (by mass) square of the velocities of the stars in the cluster, ρ is the mass density of the cluster, ⟨m⟩ is the average star mass in the cluster, G is the gravitational constant, and N is the number of stars in the cluster. The derivation of this formula involves analysis of 2-body interactions and is ubiquitous on the internet.

Because of the higher densities ρ within the cores of globular clusters, estimated core relaxation times should be shorter than relaxation times for the outer, less dense, parts of the cluster, as well as for the clusters as a whole. Estimated relaxation times for the central regions of globular clusters are on the order of one million to a hundred million years,¹⁰ and N-body computer simulations suggest that central rotation should be erased after a few relaxation times.¹¹

To make matters worse for secular astronomers, some physicists and astrophysicists have long argued that “self-gravity significantly speeds the relaxation of stellar systems through excitation of large-scale modes.”¹² Because the classically-derived equation ignores self-gravity and other effects, they argue that it is underestimating relaxation rates, thereby overestimating cluster relaxation times.^13,14 If true, this would make the core relaxation times even shorter, putting still more stress on the secular story. However, some of these scientists seem to have reversed themselves, recently arguing that these other effects do not make much of a difference to relaxation times, at least in some cases.¹⁵

Contradiction between rotation signatures and ages of globular clusters

In any case, there is an apparent contradiction between these central rotation signatures and the presumed ages of these globular clusters. The accompanying press release said that none of the current theoretical models (as of 2014) predicted “such a ubiquitous and strong rotation”.⁸ A phenomenon called ‘core collapse’ can erase rotational signatures, but none of the 11 globular clusters in the study are thought to have undergone core collapse.

Needless to say, the authors of the study were very surprised. Lead author Maximilian Fabricius, said the result was ‘astonishing’.⁸ Co-author Eva Noyola of the University of Texas was quoted as saying:

“Theory and numerical simulations of globular clusters indicate that any central rotation should be erased on relatively short timescales. … Because these globular clusters were formed billions of years ago, we would expect that any rotation signature would have been eradicated by now. Even though previous measurements showed some rotation in a handful of systems, they only probed the motion of stars in the outer regions.”⁸

As best as I can tell, the only reference in the creation literature to this phenomenon is in a couple of popular-level articles I wrote.^16,17 I am a little surprised that this hasn’t received more attention in the creation astronomy community.

Since then, evidence of internal rotation in globular clusters has increased. In 2018, one survey found strong evidence (>3σ confidence level) of rotation in 13 out of 22 globular clusters,¹⁸ and a second survey found strong evidence (>3σ confidence level) for internal rotation in 11 out of 51 globular clusters, as well as 11 more clusters having evidence of internal rotation at the 2σ confidence level.¹⁹ At least seven papers have reported such internal rotation results since 2014.²⁰

Conclusion

This is evidence that at least some globular clusters are much younger than their secular age assignments. Of course, if God used some kind of time dilation effect to get distant starlight to us quickly, these globular clusters could still be millions of years old as measured by clocks in deep space. But it’s just one more indication that, even in a possibly time-dilated universe, secular age assignments are still greatly inflated.²¹